There are two parts to the calculations on who gets to fight in a round of battle, 1) the Combat Unit Quota, and 2) the Quota Cost.
The best references I know of, on how these work are here:AGEWiki: FrontageFrontage Primer
Calculating the CUQ depends on Leadership with a [Terrain x Weather] table matrix. This gives you a numeric value
, the CUQ.
Now you can start to fill the CUQ with units, but
again depending on a [Terrain x Weather] table some elements will use up more or less of the CUQ than others. Just as an illustration (this has little to nothing to do with the numbers of real game situations)
this illustration assumes a CUQ of 20
referencing a [Terrain x Weather] table I've invented just for illustrative purposes, for clear weather in clear terrain, we find the Quota Costs:
Light Infantry = 3
Line Infantry = 5
Militia = 7
For each Light Infantry element chosen to be put into Frontage, 3 is subtracted from the CUQ.
For each Line Infantry element chosen to be put into Frontage, 5 is subtracted from the CUQ.
For each Militia element chosen to be put into Frontage, 7 is subtracted from the CUQ.
This given, you could get 6 Light Infantry in into frontage, or 4 Line Infantry, or 2 Militia, or a combination of these, as long as the math works.
But frontage is not filled based on how many elements you might get into frontage. Units are picked on their combat readiness, basically their Power for Offensive or Defensive depending on posture, withTQ (Quality and cohesion) playing large roles. A unit it picked to fight, and it's QC is subtracted from the CUQ. Then the next is picked, etc, etc, until the frontage is filled, or all units are fighting, which ever comes first.
The Quota Cost tables have higher costs for an element type type the worse the terrain and weather is, thus reducing frontage. So basically, the faster an element is, with regards to terrain and weather, the more of them that can get into a fight at one time (round of battle).