Use for frontend architecture boundaries: code ownership, directory roles, file placement, import direction, UI/component extraction, API/data contracts, fetching/query, URL state, mappers/schemas/DTOs, hooks/stores, architecture docs, and tool enforcement. Also use before agent-initiated architecture-enforcement changes in lint, CI, import-boundary checks, or similar automation—even without a user request—and for small UI changes that may leak business rules or external data into lower-level code.