the difference is exactly as it implies. a multi pilot a/c requires more than 1 pilot to operate it, while multi crew a/c refers to the situation where a pilot is backed up in whatever role by another aircrew type, nav for example. with regard to civvi licences the distinction is made as it is recognised that the working relationship between a tristar front end crew (for eg) is the same as that of a BA crew, while a tornado crew, pilot and nav, is a completely different situation in terms of 'pilot operating procedures'.