Butts in the air usually means scenting. They'll do this after a disturbance (e.g. hive inspection), when the bees need to find their way back in to the hive. They'll crack the last segment of their abdomens (and if you look closely, you can see how the last segment doesn't line up with the rest of the abdomen), stick their butts in the air and waft the Nasonov pheromone into the air to tell foraging bees this is the way home. This is scenting:
https://mudsongs.org/honey-bees-scenting/When they're fanning for temperature regulation, etc., they'll hold tight to the ground, bend their bodies inward and beats their wings like crazy, and they'll often line up behind each other while they're doing it.
Sometimes the bees will fan
and scent, butts up and down.
I saw a documentary series on Netflix, The Life of Bees, something like that, where they describe honey bees fanning, and as far as I know, they get it backwards.