!!! Overview
[{$pagename}] is defined in [RFC 7231], section 6.3.5: 204 No Content, as a [HTTP Status Code] to indicate the success status response code indicates that the [request] has succeed, but that the [client] doesn't need to go away from its current page. 

[{$pagename}] [response] is cacheable by default. 

An [Etag] [HTTP Header Field] is included in such a [response].

A [{$pagename}] [Result Code] does not indicate whether this lack is temporary or permanent. If the server knows that this condition is likely to be __permanent__, a [HTTP 410] (Gone) should be used instead of a [{$pagename}] status.

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