Pretty sure it's not used for anything.
Do you know how can I use it? like for example, I want to add some special move by pressing 'slot 1' key in the keyboard.Can I define which button will be slot 1?
You can only bind keys on the client side.
The only thing you could really do is have a client sided mod with a keybind and then have the client call a server command when they push the key and then have the server command call .onTrigger with the proper arguments. But then it's essentially useless to have the server command call .onTrigger because you have the server command being called so you can just use that instead.
Why not just - make a new client-sided add-on which adds specific key for 'slot 1', and when someone will make an add-on which includes the use of 'slot 1' key, he will tell everyone to download that add-on?
You could but it'd accomplish the same thing if you had the keybind the client uses call a server command and just use that, instead of having the server command call .onTrigger.
What are you trying to accomplish?
I was working on my blocko shooter I made like 4 years ago and I added the option to sprint ("crouch" key which is slot 3) and the option to crouch (which was "jet" slot 4). I don't want to use slot 4 since it's for aiming in weapons, but i need a new trigger for my crouching ability.
You could use the brick plant key, the light key, or something like that.
Are you sure it's possible on player-types? I know the light key works for reloading in weapons (anyways I need the light key for the reload). If so, Can you write a working command which uses the plant key? or an add-on you know which use that?