The file format, physical medium, or dimensions of the resource. (ISO15836) Best practice is to use a controlled vocabulary such as the list of Internet Media Types [MIME] provided here: http://www.iana.org/assignments/media-types/
An unambiguous reference to the resource within a given context. (ISO15836)
A related resource from which the described resource is derived. (ISO15836)
The spatial or temporal topic of the resource, the spatial applicability of the resource, or the jurisdiction under which the resource is relevant. (ISO15836)
Information about rights held in and over the resource. (ISO15836)