Return an uniqueID of the given unit as string if it's player and empty value if it's AI.
The unique ID may be up to 100 characters long and may contain numbers, uppercase letters and underscores (the value can be used to construct a valid variable name by appending to another valid variable name).
Note: while for ArmA 2 or ArmA 2: Operation Arrowhead until version 1.60 the result always contains a number, it should be always handled as an opaque string. Converting it to a Number can cause precision problems, as Number cannot accurately represent integers above 16777216 (2^24). Moreover, game versions since ArmA 2: Operation Arrowhead 1.61 provide a non-numeric value as a result.
In Arma 3 the returned string is the steamID64 of the player.
Returned
String in format ("_SP_AI_", "_SP_PLAYER_" in SP, "" or UID number string in MP)