
I have a two year old that has won out of Post Graduate (our entries have been low, in age classes and the first adult class which for us is usually PG) but isn't to my mind ready for it.
So actually you would move your dog to Limit when it has won out of Post Graduate.
Just occasionally you may have an exhibit whose age and maturity (say 3 plus) would make it look odd in Post Graduate, it would have the quality to compete higher but perhaps was little shown, or an exhibitor may have littermates, and have to put one in a higher class.
Or if your like me and your dog has been doing well in Post Graduate and you know they may win out soon, you like to enter a second class to test the water at the next level.