This is a guide on how to make trigger actions that only appear executed for certain players (e.g. text only popping up for one person, like a "private message").
Firstly, You will want to use with the desired variable (you can leave parameter B as 0) on a trigger, it only has to execute once.
Then, on another trigger, use with the variable in parameter A and specified player slot in parameter B to limit the action types below it. Also, the player slots go as 0, 1, 2, etc. so the number will always be one less than the slot number.
You can use this to make private chat messages only seen by (a) certain player(s), changing gravity for everyone except for one player, etc. But note that it must target specific people, so if you want it to be able to favor player-initiator, you will have to create another system so it can change the slot it must equal depending on who presses the button.