In case others find this thread and get confused.........
John just mentioned the "DB Bus system". That isn't what the pass is about. The pass covers "DB IC Bus" and the IC bit is important, these are long distance and often international links which connect with the DB long distance train network. They are infrequent by German standards (bear in mind the standard pattern for IC trains is hourly....) and there are only a few routes. They are white with red stripes like their long distance trains. They are what in real english we would call "coaches".
But DB are the biggest operator in Germany (possibly Europe) of normal buses, which form quite a dense network covering all the little villages and tourist stops where the trains don't reach. These are red and normally have a DB logo, although sometimes they manage the system on behalf of local authorities and subcontract to other bus firms who may display a notice with a DB logo. These buses (maybe 98% of the DB Bus system) are not mentioned in the small print of validity for the German Rail Pass - but will be offered by the DB journey planner if you don't deselect Bus