Relative Quality Factor in HTTP Accept Header

Relative Quality Factor specifies what language or media type the user would prefer, on a scale of 0 to 1. Each language-range/media type range MAY be given an associated quality value which represents an estimate of the user's preference for the languages/media types specified by that range.

The quality value defaults to "q=1".

For example,  Accept-Language: en-gb, en;q=0.7 would mean: "I prefer British English and accept other types of English."

For media type, if the Accept header is Accept: text/plain;q=0.9, text/html, which declares that the client can accept media types of text/plain and text/html but prefers the latter.