Some conferences have special selection parameters written into their contracts with specific bowls — for example, the
Citrus Bowl is contractually obligated to select the winningest Big Ten and SEC teams that do not make a CFP game (semifinal or New Year's Six Bowl), or a team within one win of the winningest in its conference. The MAC's bowl contracts require that both division champions, if eligible, receive bids to one of its five contracted bowls.
Sorry, I know that Wikipedia isn’t reputable, but it’s the only thing I can find right now showing how they actually are required to select.
en.m.wikipedia.org