'Unit naming conventions are spelled out in the Army field manuals and regulations.

Since the Marines still have regiments as tactical formations, the way they differentiate between battalion and regiment is different from the Army.

Army: 1/2/A/2-5 IN = 1st Squad, 2d Platoon, A Company, 2d Battalion, 5th Infantry

Marines: 1/2/A/2/5 Marines = 1st Squad, 2nd Platoon, Company A, 2nd Battalion, 5th Marines'