From Media Fragments Working Group Wiki
This is an HTTP request header which allows the media fragment to be transmitted verbatim to the server.
When a Fragment request is handled by a compliant HTTP server, it is acknowledged by the response header:
the value of which is a serialization/canonicalization of the actual fragment provided by the server.
This must also be accompanied by a Vary: response header for Fragment:
Interaction with Range
If part of the fragment is converted to a Range request header, that portion MUST NOT be included in the Fragment request header. See HTTP_Examples#Conversion_of_Fragment_to_Range for examples.
Handling of fragment identifiers in redirected URLs http://ftp.ist.utl.pt/pub/drafts/draft-bos-http-redirect-00.txt