Account identifier. Newly created V3 Accounts use a 22-character Wingspan ID; migrated Accounts may retain a 24-character lowercase Mongo ObjectId.

Select the Account for an Account-scoped operation. A direct ServiceAccount API key MUST supply this header, and the target must be within the ServiceAccount owner's or Authorization grant's Account boundary. A Person bearer may select an Account on which it has an active Stakeholder, and an Account session may select its bound Account (or a descendant only when the session explicitly includes descendants).