link: https://atproto.com/specs/nsid tags:

• ATProtocol

Namespaced Identifiers (NSIDs) are used to reference Lexicon schemas for records, XRPC endpoints, and more.

The basic structure and semantics of an NSID are a fully-qualified hostname in Reverse Domain-Name Order, followed by a simple name. The hostname part is the domain authority, and the final segment is the name.