Overwatch

In modern warfare, overwatch is the state of one small unit or military vehicle supporting another unit, while they are executing fire and movement tactics. An overwatching, or supporting unit has taken a position where it can observe the terrain ahead, especially likely enemy positions. This allows it to provide effective covering fire for advancing friendly units. The term overwatch originates in U.S. military doctrine.

UNSC Bases have Marines on overwatch, providing visuals on any inbound enemy ground or air forces, working in tandem with external security cameras.