Sets the main unit for display of floating combat text.

Certain types of floating combat text are only displayed for the "active" unit (normally the player): incoming damage, incoming heals, mana/energy/power gains, low health/mana warnings, etc. This function is used by the default UI to allow the player's vehicle to "stand in" for the player for purposes of combat text; using this function with units other than "player" or "vehicle" has no effect.