From Halopedia, the Halo wiki
UNSC Marine Corps
Repel the enemy's Assault by fire and close combat
S4/BLCE/1/7 is a UNSC Marine Corps infantry unit .
They fought during the Battle for Earth, in battles such as the Battle of Voi. Later, elements of the unit fought during the last battle of the Human-Covenant War, the Battle of Installation 00.
The unit is first seen on the left side of an M274 Ultra-Light All-Terrain Vehicle, where the unit is named.