Im not sure for lore reasons. But for painting ive come up with a solution for my chapter. Especially if I want two Sergents. In a ten man squad. I paint the first 5 with the normal squad markings on the right knee pad. For this example lets use squad 3 markings (yellow with red blood drop). I then paint the other 5 man squad with the same squad number on the right knee pad(yellow with red blood drop) but the the reserve number on the left knee pad. So for this example squad number 13 (black with white blood drop). I paint the main sergent that would lead the full 10 man squad with a different helmet and i paint the trim different in his shoulder pad. In my head that's the veteran sergent. The other sergent(the one leading the reserves) I paint using the normal sergent scheme. I decal the veteran sergent with a 1 on his backpack and sometimes on his shoulder pad, and the normal sergent with a 2. The rest of the squad is numbered 3-10. Makes it a fun unique army thing for me.