mdb is the format for earlier versions of Access (97, 2000, 2002, 2003). If you open the mdb and edit it to include any features not available in earlier versions and try to save the database as mdb it will give a message (to the best of my knowledge). If you choose to save the edited database in accdb format, it will save --but you will not be able to open the accdb database with a pre-2007 version of Access.
Not sure if that's exactly what you asked.