!!! Overview
[{$pagename}] 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 ":".

!! [RFC 7320] 
As a part of the [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 [{$pagename}], 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).

!! More Information
There might be more information for this subject on one of the following:
[{ReferringPagesPlugin before='*' after='\n' }]