It varies a lot by university. In Osnabrück the Semesterticket basically allowed to travel as far as Hanover, Münster and Bremen, which was pretty far as that goes. In Rostock on the other hand it only covered the city and a few of the surrounding villages (back in 2011, though).

I think there was a provision of opting out if you needed a regular public transport ticket anyway to get to the uni (because you lived too far outside) but that varies by uni as well.