Media Query evaluation of properties appears to be undefined

The Media Queries Proposed Recommendation does not appear to define when a media property expression
(such as "min-width: 200px") evaluates to true and when it evaluates to false.

Some specific conditions (unknown feature/property, unknown unit, value feature) are defined, but I
can't find an explicit statement of when a media property expression evaluates to true.

Regards,
Maciej