S4/BAG/2/5
From Halopedia, the Halo wiki
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S4/BAG/2/5 was a UNSC Marine Corps unit. It was stationed on Earth in 2552.
Background
That year, they were assigned to Sol Defense Group 3/A-5 when the group requested rearmament[1].
Speculation
It is possible the S stands for the company, Sierra. The 4 may be fourth platoon of S Co, or fourth battalion that the company is part of. BAG could stand for Brigade Assault Group, which would be the infantry element of a reinforced Marine regiment, or Marine expeditionary brigade. The 2/5 could either be 2nd Battalion, 5th Marine Regiment, or 2nd Regiment, 5th Division. This is simply conjecture and should not be taken as official.
