vocabularies

The governed set of CREDENTIAL types a BUYER must present to complete a purchase — the CATEGORY of gate, never the specific document. Region-neutral by construction (the Pool is universal): a US FFL / firearms licence, a controlled-chemicals or explosives-precursor permit, and a professional-purchase credential (a pesticide-applicator or medical-device certificate) are all the ONE `licence-required` type — the specific licence is a per-instance label the seller states (A16: the same 'licence' word names a different document in each jurisdiction, so the specific type is a literal, never a member here). A medical `prescription-required` is a distinct, universally-attested credential (a doctor's authorisation, not a commercial licence). Distinct from the age gate (governed by age_restriction, compliance, b139 — retail alcohol/tobacco to a consumer is an AGE gate the SELLER's licence covers, not a buyer credential, so it is NOT a member here) and from the buyer-CLASS gate (governed by customer_type_eligibility): those are respectively a banded age and a persistent buyer segment, whereas a purchase gate is a per-transaction document the BUYER must hold and have verified. A growing set (L13): a credential type not yet listed is deferred pending its first real instance.

Answers the question: purchase gate.