Whilst looking into game-oriented applications of programming patterns I discovered a question on this site which suggests that a 'set of states' is superior to a 'stack of states' but does not clarify how a 'set of states' would be applied.
A stack of states is simple enough to comprehend - the state on the top is the active state.
But what exactly is a set of states? A set as I understand it is merely a collection of unique objects (i.e. there are no duplicates). Presumably all the states in the set are currently active. But sets are unordered, so how does one maintain order in an unordered collection? An ordered set could be used, but that then brings the question of why use a sorted set and not a list - what benefit does the uniqueness bring? I'm just not sure I can imagine how such a thing would be implemented or what benefits it would bring. (Presumably the former issue is causing the latter).
So what exactly is a 'set of states'? (In the context of game design/programming).