From Halopedia, the Halo wiki
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 Marines, or 2nd Marines, 5th Division. This is simply conjecture and should not be taken as official. It is also possible that "S4" refers to the designation for the logistics element of a battalion-sized element. The whole unit code could be "S4 (or Logistics Section)/2d Battalion/5th Marine Regiment". This style of designation has been in use by the real life United States Marine Corps.
Halo: Evolutions suggests that the first part of the designation refers to the planet the unit is garrisoned on. It gives the example as "E2/BAG/1/7", where "E2" stands for the colony world of Eridanus II. Based on this piece of information, it would appear that "S4" stands for Sol IV - Mars. Continuing with this, the "2/5" may mean "2nd Battalion, 5th Marines", as what was previously suggested as a possibility above.
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