That's just when you put it all back together you stick your feeler gauge between the snap ring and everything it holds down and there needs to be a certain amount of clearance so the clutches can release if they're too tight then they'll rub on each other and drag I reckon if they're too loose they'll never engage LOL.. I've never checked it...
Remember the first 904 rebuilt I was all worried about the end play until AAR cuda said he's never checked and play before and rebuilt numerous transmissions for drag racing...