ifc-syntax-data-types

name	ifc-syntax-data-types
description	Use when you need to pick or read the value type of an IFC attribute or property : which measure type a length, area, or volume needs, when to use IfcLabel vs IfcText vs IfcIdentifier, IfcBoolean vs IfcLogical, and how a typed value is wrapped in a STEP file. Prevents storing a quantity under the wrong measure type, exceeding the 255-character string cap, collapsing a three-valued logical into a boolean, and writing a bare value where a select type requires a TYPENAME() wrapper. Covers the IfcValue / IfcSimpleValue / IfcMeasureValue / IfcDerivedMeasureValue select hierarchy, the measure types, string and identity types, the USERDEFINED / NOTDEFINED enumeration convention, refinement WHERE rules, and IFC2x3 / IFC4 / IFC4.3 version differences. Keywords: IFC data types, IfcValue, IfcSimpleValue, IfcMeasureValue, IfcDerivedMeasureValue, IfcLengthMeasure, IfcAreaMeasure, IfcVolumeMeasure, IfcLabel, IfcText, IfcIdentifier, IfcBoolean, IfcLogical, measure type, typed value wrapping, USERDEFINED, NOTDEFINED, ObjectType, what type should this value be, wrong units in my model, my area is off by 1000x, string too long, how do I store a property value, IFC2x3, IFC4, IFC4.3.
license	MIT
compatibility	Designed for Claude Code. Requires IFC IFC2x3, IFC4, IFC4.3.
metadata	{"author":"OpenAEC-Foundation","version":"1.0"}

IFC Syntax : Data Types

This skill answers one concrete question : what can the value of an IFC attribute or property hold, and how is that value typed in a file? It covers IFC's own resource- layer value-type system : the named defined types (IfcLengthMeasure, IfcLabel, IfcBoolean, ...) and the SELECT unions that group them.

Applies to IFC2x3, IFC4, and IFC4.3. Member lists differ by version : see the Version Differences section.

Quick Reference

Two type systems : keep them apart

IFC has two layered type systems. ALWAYS keep them separate :

1. The EXPRESS meta-language (ifc-syntax-express) : the constructs TYPE, ENTITY, SELECT, ENUMERATION, the base types STRING, REAL, INTEGER, BOOLEAN, LOGICAL, BINARY, NUMBER. These are the blocks the schema is written with.
2. IFC's own value-type system (this skill) : the named defined types IFC declares on top of those base types, plus the SELECT unions that group them.

IFC wraps every base type in a named defined type because an EXPRESS SELECT may list only named types, NEVER raw BOOLEAN / REAL / INTEGER. That is why TYPE IfcBoolean = BOOLEAN;, TYPE IfcReal = REAL;, and TYPE IfcInteger = INTEGER; exist : so they can be members of a select.

IfcValue : the two-level union

IfcValue is the most general value type in IFC. It is the declared type of IfcPropertySingleValue.NominalValue and of IfcMeasureWithUnit.ValueComponent. Verified verbatim (IFC4.3, introduced IFC2x) :

TYPE IfcValue = SELECT
	(IfcDerivedMeasureValue
	,IfcMeasureValue
	,IfcSimpleValue);
END_TYPE;

IfcValue is a SELECT whose three members are themselves SELECT types : a two-level union. A parser resolving an IfcValue descends through one branch to reach a concrete defined type.

Branch	Holds	Has a unit?
IfcSimpleValue	primitive, dimensionless values : booleans, integers, reals, strings, logicals, dates, binary	No
IfcMeasureValue	base physical measures : length, area, volume, angle, mass, count, ratio	Yes (single SI dimension)
IfcDerivedMeasureValue	compound measures : force, pressure, velocity, density, thermal transmittance, money	Yes (derived SI dimension)

IfcSimpleValue members (IFC4.3 : 15 members)

Type	Base	Use for
IfcBoolean	BOOLEAN	a known two-valued yes/no fact
IfcLogical	LOGICAL	a fact that may also be UNKNOWN
IfcInteger	INTEGER	whole numbers
IfcPositiveInteger	INTEGER, SELF > 0	strictly positive whole numbers (IFC4+)
IfcReal	REAL	dimensionless real numbers
IfcLabel	STRING(255)	a human-readable name
IfcText	STRING (unbounded)	free-form description / note
IfcIdentifier	STRING(255)	a machine-readable code / key
IfcBinary	BINARY	embedded binary data, for example textures (IFC4 ADD1+)
IfcDateTime	STRING, ISO 8601	date and time (IFC4+)
IfcDate	STRING, ISO 8601	calendar date (IFC4+)
IfcTime	STRING, ISO 8601	time of day (IFC4+)
IfcDuration	STRING, ISO 8601	a duration (IFC4+)
IfcTimeStamp	INTEGER	seconds since the Unix epoch
IfcURIReference	STRING	a URI / URL / URN (IFC4+)

IfcMeasureValue : high-frequency base measures (IFC4.3 : 23 members)

Measure type	Base	Unit type implied
IfcLengthMeasure	REAL	LENGTHUNIT
IfcPositiveLengthMeasure	IfcLengthMeasure, SELF > 0	LENGTHUNIT
IfcNonNegativeLengthMeasure	IfcLengthMeasure, SELF >= 0 (IFC4+)	LENGTHUNIT
IfcAreaMeasure	REAL	AREAUNIT
IfcVolumeMeasure	REAL	VOLUMEUNIT
IfcPlaneAngleMeasure	REAL	PLANEANGLEUNIT
IfcMassMeasure	REAL	MASSUNIT
IfcCountMeasure	INTEGER (IFC4.3 ; NUMBER before)	not a unit type
IfcRatioMeasure	REAL	dimensionless
IfcNormalisedRatioMeasure	IfcRatioMeasure, 0.0 <= SELF <= 1.0	dimensionless

Full 23-member list : see references/methods.md.

IfcDerivedMeasureValue : high-frequency derived measures (IFC4.3 : 70 members)

Measure type	Base	Notes
IfcForceMeasure	REAL	force
IfcPressureMeasure	REAL	pressure
IfcLinearVelocityMeasure	REAL	velocity
IfcMassDensityMeasure	REAL	density
IfcThermalTransmittanceMeasure	REAL	U-value
IfcMonetaryMeasure	REAL	money, currency-agnostic
IfcCompoundPlaneAngleMeasure	LIST [3:4] OF INTEGER	degrees / minutes / seconds, NOT a scalar

Full 70-member list : see references/methods.md.

Typed-value wrapping in a STEP file

A value whose attribute is declared with a concrete type is written bare : 'Wall-01' for an IfcLabel-typed Name.

A value whose attribute is declared as a SELECT (such as IfcValue) MUST be wrapped in the upper-cased name of the concrete member type, so the reader knows which branch was chosen :

#101=IFCPROPERTYSINGLEVALUE('FireRating',$,IFCLABEL('REI 60'),$);
#102=IFCPROPERTYSINGLEVALUE('NetArea',$,IFCAREAMEASURE(12.5),$);
#103=IFCPROPERTYSINGLEVALUE('LoadBearing',$,IFCBOOLEAN(.T.),$);

The wrapper name IS the type tag. NEVER write a bare value where a select type is expected : the reader cannot tell 12.5 apart as an area, a length, or a plain real.

Version differences (compact)

Item	IFC2x3	IFC4	IFC4.3
IfcSimpleValue members	7	15	15
IfcDateTime, IfcDate, IfcTime, IfcDuration, IfcBinary, IfcPositiveInteger, IfcURIReference	absent	present	present
IfcCountMeasure base type	NUMBER	NUMBER	INTEGER
IfcNonNegativeLengthMeasure	absent	present	present
IfcLabel / IfcIdentifier 255 cap	recommended	mandatory	mandatory

Decision Trees

Which value type for a property value?

What does the value represent?
|
+-- A physical quantity with a single SI dimension
|   (length, area, volume, angle, mass, ratio, count)
|   --> IfcMeasureValue branch. Pick the measure type whose
|       dimension matches : IfcLengthMeasure, IfcAreaMeasure, ...
|
+-- A compound physical quantity
|   (force, pressure, velocity, density, U-value, money)
|   --> IfcDerivedMeasureValue branch.
|       IfcForceMeasure, IfcPressureMeasure, IfcMonetaryMeasure, ...
|
+-- A non-physical, dimensionless value
    (text, name, code, yes/no, count of items, date, real number)
    --> IfcSimpleValue branch. IfcLabel, IfcText, IfcBoolean, ...

IfcBoolean or IfcLogical?

Can the attribute meaningfully be "unknown / not applicable",
distinct from FALSE?
|
+-- NO : it is always a known yes/no fact
|   --> IfcBoolean (.T. / .F.)
|
+-- YES : "not known" is a real third state
    --> IfcLogical (.T. / .F. / .U.)

NEVER substitute IfcBoolean for a schema-declared IfcLogical :
that collapses UNKNOWN into FALSE and loses information.
A present .U. is NOT the same as an absent $ (unset OPTIONAL).

IfcLabel, IfcText, or IfcIdentifier?

What is the string for?
|
+-- A short, human-readable display NAME
|   --> IfcLabel (STRING(255))
|
+-- A machine-readable CODE / KEY, stable and locale-independent
|   (classification code, part number)
|   --> IfcIdentifier (STRING(255))
|
+-- Free-form descriptive prose, a note, possibly long
    --> IfcText (unbounded STRING)

IfcLabel and IfcIdentifier are byte-identical (STRING(255)) but
NEVER interchangeable in intent : the schema picks one deliberately.

Which measure type for a quantity?

Match the measure type to the physical SI dimension :
|
+-- a distance          --> IfcLengthMeasure
+-- a distance, > 0     --> IfcPositiveLengthMeasure
+-- a distance, >= 0    --> IfcNonNegativeLengthMeasure (IFC4+)
+-- a surface area      --> IfcAreaMeasure
+-- a volume            --> IfcVolumeMeasure
+-- a planar angle      --> IfcPlaneAngleMeasure
+-- a latitude/longitude--> IfcCompoundPlaneAngleMeasure (deg/min/sec list)
+-- a mass              --> IfcMassMeasure
+-- a count of items    --> IfcCountMeasure
+-- a same-kind ratio   --> IfcRatioMeasure (percent / 100)
+-- a ratio in [0,1]    --> IfcNormalisedRatioMeasure

NEVER pick by what parses : an area stored as IfcLengthMeasure
parses fine and resolves to the wrong project unit.

Patterns

Pattern : Wrap select-typed values

ALWAYS wrap a value as TYPENAME(value) when the attribute is declared as a SELECT (such as IfcValue, IfcSizeSelect, IfcUnit). The wrapper is the discriminator. ALWAYS write a value bare when the attribute is declared with a concrete defined type : IfcRoot.Name is IfcLabel, so 'Wall-01' is written bare. NEVER write a bare numeric or string value into a select-typed attribute : the reader has no way to know which select member it is, and the file is malformed.

See ifc-syntax-step-physical-file for the full STEP instance syntax.

Pattern : Pick the measure type by SI dimension

ALWAYS choose the measure type whose physical dimension matches the quantity : a length is IfcLengthMeasure, an area is IfcAreaMeasure, a volume is IfcVolumeMeasure, an angle is IfcPlaneAngleMeasure. The measure type, NOT the number, selects the unit. An IfcLengthMeasure of 3.5 means 3.5 of the project length unit. A wall area stored as IfcLengthMeasure resolves against LENGTHUNIT instead of AREAUNIT and silently corrupts every downstream quantity take-off. NEVER decide a measure type by whether the value parses : every measure type is ultimately a REAL, so a wrong type always parses.

See ifc-syntax-units for how a unit type resolves to a concrete unit.

Pattern : Choose two-valued vs three-valued logic

ALWAYS use IfcLogical when "unknown" or "not applicable" is a meaningful state distinct from FALSE. The canonical case is IfcMaterialLayer.IsVentilated, an OPTIONAL IfcLogical : TRUE is a ventilated air gap, FALSE is solid material, UNKNOWN is a non-ventilated air gap. ALWAYS use IfcBoolean when the attribute is a yes/no fact that is always known. NEVER treat a present IfcLogical value of .U. (UNKNOWN) as equivalent to an absent OPTIONAL attribute ($) : $ means no value was supplied, .U. means a value was supplied and it is the third logical state.

Pattern : Pair USERDEFINED with ObjectType

Almost every IFC PredefinedType enumeration (for example IfcWallTypeEnum) ends with two special members : USERDEFINED and NOTDEFINED. NOTDEFINED means no predefined type applies. USERDEFINED means a type outside the fixed list.

ALWAYS supply the paired free-text type name when PredefinedType = USERDEFINED : on an occurrence object, in IfcObject.ObjectType (an IfcLabel) ; on a type object, in IfcElementType.ElementType. An entity WHERE rule (for example IfcWall.CorrectPredefinedType) enforces this pairing. NEVER set PredefinedType = USERDEFINED and leave the paired attribute unset : the file violates the WHERE rule and is schema-invalid.

An enumeration value is NOT an IfcValue. IfcValue covers user-supplied data values ; a PredefinedType is a schema-fixed classification token. They are different layers. See ifc-syntax-express for the ENUMERATION construct.

Pattern : Respect refinement WHERE rules

The Positive*, NonNegative*, and Normalised* types are NOT separate primitive types : they are defined types layered on a base measure with an added WHERE rule.

Type	Base	WHERE rule
IfcPositiveLengthMeasure	IfcLengthMeasure	SELF > 0.
IfcNonNegativeLengthMeasure	IfcLengthMeasure	SELF >= 0.
IfcPositivePlaneAngleMeasure	IfcPlaneAngleMeasure	SELF > 0.
IfcPositiveRatioMeasure	IfcRatioMeasure	SELF > 0.
IfcNormalisedRatioMeasure	IfcRatioMeasure	{0.0 <= SELF <= 1.0}

ALWAYS check the refinement's WHERE rule before assigning : 0.0 or a negative value in an IfcPositiveLengthMeasure parses as a plain real but is schema-invalid. NEVER assume a value that parses satisfies the type : the WHERE rule is part of the type.

Pattern : Honour version differences

ALWAYS target the member list for the file's declared schema version :

• In IFC2x3, IfcSimpleValue has only 7 members. NEVER emit IfcDateTime, IfcDate, IfcTime, IfcDuration, IfcBinary, IfcPositiveInteger, or IfcURIReference as an IfcValue payload in an IFC2x3 file : IFC2x3 uses structured date entities (IfcCalendarDate, IfcLocalTime, IfcDateAndTime).
• In IFC4.3, IfcCountMeasure is an INTEGER. A non-integer count is valid IFC4 but invalid IFC4.3. ALWAYS use whole numbers for IfcCountMeasure in IFC4.3.
• The IfcLabel / IfcIdentifier 255-character cap was advisory before IFC4 and mandatory from IFC4 onward.

See references/methods.md for the complete per-version member tables.

Reference Links

• references/methods.md : the complete verified member lists : IfcSimpleValue (15 + the IFC2x3 7), the full 23-member IfcMeasureValue select, the full 70-member IfcDerivedMeasureValue select, every base definition, the measure-to- unit-type mapping, and the per-version difference tables.
• references/examples.md : worked scenarios : typing a property value, reading a wrapped STEP value, choosing between string types, USERDEFINED pairing, IFC2x3-vs-IFC4 value typing.
• references/anti-patterns.md : the failure modes this skill prevents, each with the reason it fails.

Official sources

• IfcValue : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcValue.htm
• IfcSimpleValue : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcSimpleValue.htm
• IfcMeasureValue : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcMeasureValue.htm
• IfcDerivedMeasureValue : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcDerivedMeasureValue.htm
• IFC2x3 IfcSimpleValue : https://standards.buildingsmart.org/IFC/RELEASE/IFC2x3/TC1/HTML/ifcmeasureresource/lexical/ifcsimplevalue.htm

Related skills

• ifc-syntax-express : the EXPRESS meta-language : TYPE, SELECT, ENUMERATION, WHERE, the base types this skill's defined types are built on.
• ifc-syntax-units : how a measure type's unit type resolves to a concrete unit via IfcUnitAssignment.
• ifc-syntax-property-sets : where IfcValue-typed property values are carried.