I don't know how it works on the TeleAtlas discs. On the old NAVTEQ CDs (the whole US was on 1 DVD, so it didn't affect that), the other CDs had major highways for the whole country, and a somewhat higher level of detail (but nowhere near complete) for slightly out-of-region areas.

A good portion of the size of the data is info about each road - speed limits, turn restrictions, 1-way, and so on. Even on an out-of-region disc, you'd still see the map image with roads, rivers, etc. but most roads would be shown in gray, not brown, as the GPS just knew them as features, not roads. If you turned from a brown road onto a gray one, you'd get a blue "Correct dest. guidance not available" overlay on the map, and if you were currently being guided by the nav, it would announce "You are not on a digitized road".

I assume that something similar happens with the TeleAtlas discs. I'm traveling now, but when I get back to New York I'll pop in the West DVD and see what happens.