From Halopedia, the Halo wiki
UNSC Marine Corps
Repel the enemy's assault by fire and close combat
S4/BRT/1/7 is a UNSC Marine Corps military unit. The unit fought throughout the late Human-Covenant War, in conflicts such as the Battle for Earth, the Battle of Voi, and the Battle of Installation 00.
In the Halo: Reach beta, some UNSC ammo crates were labeled with the unit's designation. This was changed for the final game.