| What is inside this system, and what is outside? | System boundary diagram | Users, third-party services, external APIs, payments, storage, or trust boundaries matter. |
| Which services/apps/stores own the behavior? | Service/data-store architecture diagram | Most backend/platform plans need this first. It shows applications, data stores, responsibilities, and communication. |
| Which modules inside one service own business rules? | Internal responsibility diagram | Use only when module ownership affects the implementation or refactor risk. |
| What happens at runtime for this scenario? | Runtime flow / sequence / dynamic diagram | Use for callbacks, retries, queues, worker handoff, agent loops, and multi-step workflows. |
| What states can this object move through? | State machine / lifecycle diagram | Use for refunds, jobs, tasks, approvals, sync, retries, and failure recovery. |
| Where does data originate, transform, and persist? | Data flow / provenance diagram | Use for forms, extraction, agent outputs, imports, audit trails, and generated content. |
| Who owns each action across roles or systems? | Responsibility lane diagram | Use when humans, agents, services, or workers share a workflow. |
| Which paths are allowed, denied, blocked, or partial? | Path matrix | Use for permissions, policy, validation gates, fallback paths, and risk coverage. |
| What proves this plan works? | Validation matrix | Use in every plan. Each risk or invariant should have at least one validation row. |