Who can say? Since the definition of an uiscript is like this:
"These are game-specific internal actions that cannot be effected by .menu text alone. These are actions carried out in the mod's C code, but triggered from the menus. In a way, they are like hooks for actions into the mod from the menus. Mod makers can create their own uiScript components; the pattern is in ui/ui_main.c, UI_RunMenuScript()."
They can do pretty much anything. Maybe the startgame does indeed read somewhere what actions to take, and what map to load without the loading (briefing) screen. On the other hand, it might very well be an internal function to load the first yavin map.
Frozen by ICARUS