S4/BAG/2/5
From Halopedia, the Halo wiki
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 canon.
another possible speculation is they were assigned to Sol Defense Group 3/A-5 butwhen the group requested rearmament they where Called Sol Defence Group 4 as for the BAG See above And A-5 A might mean Addition to SDG3 but was made its own Group as second of 5 with SDG3 Being the first. This is simpley a speculation
