The SteamAppId is the unique ID Steam uses to represent a specific game. Proton uses the variable $SteamAppId for it. steamtinkerlaunch used $AID instead.

When steamtinkerlaunch was started from steam (by starting a game via steamtinkerlaunch) $SteamAppId is always set.

When steamtinkerlaunch was started from Command Line though, it tries to

• get it from the last played game
• from Command Line arguments