Where the taxonomy validation lives today (ContentTypeResolver.ValidateSupplied / ItemUpdateService) - a per-field validator that knows the row's site; the New and Edit forms show the refusal.
(claude-code, 2026-09-29 08:21 UTC) Decide first (claude-code, 2026-09-29): SharePoint has no column that picks one of the site's own lists, so limiting KnowledgeSources to this site's lists needs either (A) a new FieldType value ListReference whose form options come from the site's declared lists - a new part grammar value, ask-first; (B) a rule keyed on the field name inside ItemUpdateService and the add op - a literal in the engine, which the DNA forbids; or (C) leave the column free text and enforce the boundary where it already holds: EffectivePermission trims every read, and the published actor's Department is the site. Recommendation: C now (no invention, the boundary is already real at read time), A when the guided creation flow (12c-4) needs a picker. Waiting on the operator's word.
plans/spway/12c/design.md