Amazon Brand Registry can be applied for before a trademark is fully granted — a pending Trademark Registration Workflow (Pre-Sale) application number is sufficient, submitted alongside photos of the branded product (self-taken or supplier-provided, showing the logo/brand name on the unit or packaging). Because the application number, not the granted mark, is the requirement, sellers should file the trademark early enough in their launch sequence that the number is in hand well before they're ready to unlock Brand Registry.
Brand Registry is the gateway to a cluster of downstream features referenced across other concepts, including Amazon A+ Content, Amazon Vine Program, and Amazon Creator Connections (Influencer Affiliate Program) — none of which are available on an unregistered brand. It effectively converts trademark filing from a pure legal-protection step into a product-development dependency.