http://www.ietf.org/rfc/rfc2077.txt
The purpose of this memo is to propose an update to Internet RFC 2045 to include a new primary content-type to be known as "model". RFC 2045 [1] describes mechanisms for specifying and describing the format of Internet Message Bodies via content-type/subtype pairs. We believe that "model" defines a fundamental type of content with unique presentational, hardware, and processing aspects. Various subtypes of this primary type are immediately anticipated but will be covered under separate documents.