The :Once function was create to simplify this pattern

once
   # do stuff once
endonce

Once/EndOnce was created to simplify this pattern

if (<-hasRun !)
   true ->hasRun
   # do stuff once
endif

In other words, both of these are equival4ent to an `if` statement with the variable managed internally.

On the first “update” of a script the once callback will be called. So this happens before the first call to the script, but after the game is unpaused. The status variable that tracks if “:once” has been called gets set at this time. That means if you manually call :once in awake or gameloaded, it will get called again on the first run of the script.¹⁾

Note: The internal status variable for each “once” in your code is implemented as an array of bools. The position of the once command in the compiled code is the position in the once array. Upon execution of a “once”, it's status variable in the array is set to true.

For units, this happens at the end of the load process when a Unit is being loaded from a save. When a unit is being created during a mission it happens at the end of that creation process right after the script is compiled. For global scripts, it happens on the first frame (ignoring game paused status). That will be at the top or bottom of the frame depending on if the global script is set to pre or post. In other words, “Awake” happens very soon after any script is instantiated.

: Gets called for pre global scripts, unit, and post global scripts (in that order) at the top of the first (and only first) frame after a game is loaded. So this is after units are loaded, but before any game logic has executed. It happens once and only once per game load. Launching a mission counts as a “load”.