Your mode of description makes it a lot easier to discern between an ACF set and an class than it does between an ACF set and a PrC. I would say that if one starts out making an ACF and end up bunching (or wanting to bunch) the levels that the alterations happen at, then one may be looking at a PrC. The same can be said for some types of hybridization (in which case you might want to be looking at a dual-progression type PrC).