Optional brand scope. Bound as nullable so "not supplied" is distinguishable from "supplied as 0": the copilot and sibling AI services omit it entirely and must keep their account-wide read, while a brand-scoped caller that computes a 0 has a bug and should be told so rather than silently widened. Supplied and positive scopes the read to that brand and 404s on a mismatch; supplied and non-positive is a 400.