Use when writing or changing any class, object, function, property, or constant in a multi-module Android/Kotlin project (settings.gradle(.kts) includes more than one module — :app, :core:*, :feature:*, :data, etc.). Symptoms — a top-level declaration left at Kotlin's default public visibility; an implementation class, mapper, helper, constant, or UI-state holder reachable from another module though only used within its own; reaching for public "because the caller needs it"; "should this be public or internal?".
Use when writing or changing any class, object, function, property, or constant in a multi-module Android/Kotlin project (settings.gradle(.kts) includes more than one module — :app, :core:*, :feature:*, :data, etc.). Symptoms — a top-level declaration left at Kotlin's default public visibility; an implementation class, mapper, helper, constant, or UI-state holder reachable from another module though only used within its own; reaching for public "because the caller needs it"; "should this be public or internal?".
Modularization
Declare everything at the lowest visibility that still compiles. Start private; widen to internal only when another file in the same module needs it; widen to public only when a different module actually consumes it. Kotlin defaults to public, and in a multi-module project that default is usually wrong — a public symbol joins the module's API surface: it leaks implementation across the boundary and you are then obliged to keep it stable.
Public interface as the module's API → the interface is public; its implementation class stays internal (DI-bound, never named from another module).
Mapper / helper / extension / constant / UI-state holder used only inside the module → internal (or private if a single file uses it). "Rendered by a composable in this module" does not require public — internal is visible to the whole module.
Member used only within its class → private.
Widening a symbol to public is a decision, not a default: confirm a real cross-module consumer first.
