An ETag header MAY also be returned on a metadata document request or service document request to allow the client
subsequently to make a conditional request for the metadata or service document.
The context URL describes the content of the payload. It consists of the canonical metadata document URL and a
fragment identifying the relevant portion of the metadata document.
Request payloads generally do not require context URLs as the type of the payload can generally be determined from
the request URL.
For details on how the context URL is used to describe a payload, see the relevant sections in the particular
format.
An ETag header MAY also be returned on a metadata document request or service document request to allow the client
subsequently to make a conditional request for the metadata or service document. Clients can also compare the value
of the ETag header returned from a metadata document request to the metadata ETag returned in a response in order
to verify the version of the metadata used to generate that response.