A representation header is an HTTP header that describes the particular representation of the resource sent in an HTTP message body.
Representations are different forms of a particular resource. For example, the same data might be formatted as a particular media type such as XML or JSON, localised to a particular written language or geographical region, and/or compressed or otherwise encoded for transmission. The underlying resource is the same in each case, but its representation is different.
Clients specify the formats that they prefer to be sent during content negotiation (using
Accept-* headers), and the representation headers tell the client the format of the selected representation they actually received.
Representation headers may be present in both HTTP request and response messages.
If sent as a response to a
HEAD request, they describe the body content that would be selected if the resource was actually requested.