ruma-identifiers contains types for Matrix identifiers for events, rooms, room aliases, and users.
An "identifier" is an ID string unique to Matrix resources of that type.
@carl:example.com is a Matrix user ID.
ruma-identifiers is used by several of the other Ruma libraries.
It should be used whenever a developer needs to work with Matrix IDs, because it provides additional correctness guarantees over representing Matrix IDs as simple strings.