As the title suggests:
It's couples who have changed their position, at least once. They can be bottom and top at once.
I recommend Neko x Neko, it's the best example !
^^
I think Boy Love stories authors should make more versatile couples and normalize it instead of making them all exclusively stick to one designated role as a receiver or a giver. That's the best !!! Like, both of the person are men, i think they can try at least once to change and if they dont like, communicate about it !