e.g. tracks 1-3 are foo, 4-9 are bar, 10-15 are baz.
This is especially common in classical music, where the track groups might indicate grouping of movements into one composition, or grouping of CD tracks into one movement.
For an example of both, see http://www.musicbrainz.org/showalbum.html?albumid=94190 . Here, tracks 1-12 are "The Rite of Spring", and 13-22 are "Apollon Musagete" (apparently. can't say I've heard of it before). Additionally, 1-12 further breaks down into the two movements of the Rite - namely 1-6 "Adoration of the Earth" and 7-12 "The Sacrifice".
For a non-classical example, try http://www.musicbrainz.org/showalbum.html?albumid=38027 . I forget the exact structure, but it's something like 1-6 is "Yours", 7-11 is "Mine" and 12-14 is "The Truth". Interestingly, compare that album to http://www.musicbrainz.org/showalbum.html?albumid=83713 - that's the same album, but this time they apparently merged tracks 12-14 into one, thus creating an example of TrackSections.