Assassination

From Supreme Commander Wiki

Assassination is a game mode in which to be a victor one must only destroy his opponent's ACU. It is an option set before a skirmish game.

In Assasination, some things will work that would not work in other modes. for example, a suicide mission with experimental units will work, because the game will end immediately after you destroy the ACU.

See also:
Annihilation
Sandbox (game mode)
Supremacy