UserId
https://www.lukeko.com/33/userid 1- The UserId uniquely identifies every user account on Roblox. Unlike Instance.Name of a Player, which may change according the user’s present username, this value will never change for the same account
- This property is essential when saving/loading player data using GlobalDataStores. Use a player’s UserId as the data store key so that each player has a unique key
game.Players.PlayerAdded:Connect(function(player)
print(player.UserId)
end)
https://developer.roblox.com/en-us/api-reference/property/Player/UserId