Trait: Heed your Commander

The chapters’ self-reliance has given rise to extremely adept and capable squad commanders. For a cost of +15 points, tactical squads may upgrade their Sergeant to a Veteran Sergeant with the following profile:

	WS	BS	S	T	W	I	A	LD	SV
Vet Sergeant.	4	4	4	4	1	4	1(2)	9	3+
	Equipment: Vet. Sergeants come equipped with a Close combat weapon and a bolt pistol. They may take additional purchases from the Space Marine Armory. Note that they already have Terminator Honors and may not purchase it again

Any tactical squad lead by a Veteran Sergeant may take one of the following special skill at a cost of +3 points per model: Hit and Run, Tank Hunters, infiltrate (may only take infiltrate if a transport is not purchased.)

The idea in here, was basically what it states in the trait itself. The upper command structure as laid down in the Codex has been broken or lost for one reason or another. So the sergeants have been forced to step up and become leaders while still remaining sergeants.