Difference between an AUX account and a utility account

There's no difference in they way they are treated in UseridManager (or elsewhere). The intention was that an aux account would be a more personal secondary account, whereas a utility account would be something not used in any person centric way. So for instance, if I want a second account to see how my bb courses look as a student, that's only for my use and would be an aux. If I want an account that is going to receive mail for the entire department that's not really personal and would be a utility account.