CODEX: THE KOVENANT IMPERIAL ARMY.

Vrinngrenadier Regiments.

In the Kovenant Empire, the Imperial Wehrmacht has had a policy of using supplementary Regiments to help enhance and further improve upon the fighting ability Kovenant Divisions and even entire Field armies and army groups. This was so that they may have extra manpower to perform less important tasks or to directly participate in battles themselves. Such a policy of assigning supplementary forces to entire Field armies or Army groups was a common practice and they typically take the form of Kovenant Vrinngrenadiers.

Organised into what is known collectively as "Vrinngrenadier Regiments"

As previously mentioned, Vrinngrenadiers are the elite forces of a Hive City's Landwehr militia force, which specialise in intense urban warfare and large scale defensive and offensive operations in and around a given city, all the while being able to engage enemy Infantry, armour and even mechs using nothing but Infantry and their anti tank sticky bombs and magnetic mines to either destroy or disable any given vehicle or tank.

While the vast majority of Vrinngrenadiers will not be expected to fight outside their homeworld, some Vrinngrenadiers Regiments may be selected to act as supplementary and motorized Infantry specialists to an army or army group.
And typically, at least 2 in 7 Vrinngrenadiers Regiments will get selected to be shipped off-world to support the field armies in their operation.

To those Vrinngrenadiers Regiments selected for the task, the men had mixed feelings about being deployed off-world. However, the vast majority of them would accept the opportunity to do so, as they see it as a good opportunity to get some experience and to put their equipment and training to good use fighting actual enemies of the Kovenant Empire.
As such, these Vrinngrenadiers would be very highly motivated and "energized" for lack of a better term.

The question is why would the Imperial Military implement a policy of using second-line Vrinngrenadier Regiments to act as supplementary or even frontline units in a Field Army?

The answer is really rather simple, it was so that the Vrinngrenadiers can have battle experience.
The Imperial Military wanted to give its Vrinngrenadiers the chance to taste battle, so that when the campaign is over and the conflict declared won or lost, when the Vrinngrenadiers return to their Homeworlds to resume city Garrison duties, they may carry on with that same combat experience and thus help pass it on to other units to make them more effective... Essentially, to make the Imperial garrisons stronger and far more formidable.

Another reason is that oftentimes, Vrinngrenadiers would be brought in to help train the new set of men and women who were serving their mandatory 5-year long term of service in the Garrison Forces, as such, giving this Vrinngrenadiers experience was thought to have made them good trainers and instructors, since they had first-hand experience.

"What passes for training these days?? Recruits, marching formation now!"
- Vrinngrenadiers Sargent to the new recruits.

...

Organisation.

Vrinngrenadier Regiments were organized in an identical manner to their regular Imperial Army counterparts.

The Regiments consists of:

1.) 5 Companies of 50,000 Vrinngrenadiers each.

2.) 1 Heavy Weapons Company.

3.) An organically attached Light Artillery Battery Command with 5 smaller battery commands per each Company.

4.) 2 "Miniature" Sturmgeschutz Assault Gun Companies of 500 Stug-18s each. Totalling 1000 Stugs per Regiment.

5.) 1 Maintenance and Recovery unit.
As well as 1 large Logistical unit.
Including a Funker Company.

6.) 3 Light Reconnaissance Brigades (Mostly consisting of Vrinngrenadiers with Light armoured cars, IFVs and Motorcycles)