Re: Defining the meaning of headers associated with a request body

The categorization of headers is indeed very confusing; here's my take
(not an expert)

Content-Length/Transfer-Encoding
    request and response

Content-Range
    request and response

    However, HTTP/1.1 only really described it for responses to GET requests.
    its meaning in other contexts is unclear.

Content-Encoding
    request and response

    However, there's no official way for client to know what encoding is
    acceptable by server; a request with Content-Encoding is risky.

Content-Type
    request and response

ETag
    response only.
    (It applies to the representation/entity, not the response)

Expires
    response only;
    and it applies to the response! not the representation/entity.

Last-Modified
    response only;
    applies to the representation/entity

Content_Location
    request and response
    However, it doesn't seem to be very useful in requests

Content_Language
    request and response
    However, usage of Content-Language in requests is not elaborated.


So basically, only headers that are appropriate in requests are
Content-Length/Transfer-Encoding, Content-Type.

Zhong Yu