I think they want it so that if they send an Accept header in the request that asks for json, you reply with json, instead of using a query parameter to specify

I think they want it so that if they send an Accept header in the request that asks for json, you reply with json, instead of using a query parameter to specify the format. https://developer.mozilla.org/en/HTTP/Content_negotiation https://developer.mozilla.org/en/HTTP/Content_negotiation has more details about the accept header and its use.