Class Zero
We have arrived.
Well I thought it was just for Pokéballs/Pokéball Menu is that we assume that in the gameplay that we saw the player never used a Pokéball before and the button is there from the beginning So i assume it is a hotkey for Pokéballs/Pokéball menu.
In trainer battles the hotkey is not usable and still is a Pokeball.
I just figured it's whatever you set it to be...