Overview#URI Authorities is a part of the URI Scheme that holds an optional user-information part, terminated with "@" (e.g. username:password@); a hostname (e.g., domain name or IP Address); and an optional port, preceded by a colon ":". URI Scheme definitions define the presence, format and semantics of an authority component in URIs; all other specifications MUST NOT constrain, or define the structure or the semantics for URI Authorities, unless they update the scheme registration itself.
For example, an extension or application ought not say that the "foo" prefix in "foo_app.example.com" is meaningful or triggers special handling in URIs.
However, applications MAY nominate or constrain the port they use, when applicable. For example, BarApp could run over port nnnn (provided that it is properly registered).