Pay 1 credit to perform the action in any slot. It might be available or not occupied by you.
Does that mean: a) It even CAN be occupied by myself (as in: it doesn’t matter whether it is or not) b)I MUST NOT play an action slot that is occupied by myself (as in: I am allowed to play slots that either occupied by an opponent or still unoccupied)
One: It implies that the action you want to perform is not specifically one you already occupy. This is only there to take away the limit where one might think that they can only use this action card on a space they already are.
Two: The availability of the action has no limit on the action you can choose to take.
However, it is unclear to me as of yet, if you can take one that is occupied by another player.