Design / Content-Range (Internet Draft combs-http-indeterminate-range RFC 7233)
The Content-Range entity-header is sent with a partial entity-body to specify where in the full entity-body the partial body should be applied.The "Content-Range" header field is sent in a single part 206 (Partial Content) response to indicate the partial range of the selected representation enclosed as the message payload, sent in each part of a multipart 206 response to indicate the range enclosed within each body part, and sent in 416 (Range Not Satisfiable) responses to provide information about the selected representation.