Starting with AACW 1.09 or NCP 1.03, a new parameter is supported by models: CbtSignature.
When an element fires in combat, it has first to pick an enemy unit in the enemy stack engaged against its parent stack.
But which unit? For now, the chance to be picked for a unit, was strictly dependent of its health (except that units which has no line elements always get picked second if units with lines elements exist).
Here comes CbtSignature. CbtSignature alters the perceived health (which is a weight aka a chance to be picked here) of each element within a candidate unit.
What it is all about? CbtSignature is made so that elements and thus units can be less targeted by enemy fire, because of their nature. This is a relative chance, as a poor soul have to be picked anyway, so if it is not you, it will be your comrade!
This parameter was added to diminish the probability that attacking cavalry can be fired upon for NCP, as a charging cavalry is not in the front line for the same period of time compared to a line infantry firing volleys upon volleys. The recommandation is thus to put a 50 value (50%, the default is 100) cbtSignature to all cavalry. In the XLS file, NULL will make the value as being the default (100).
What about support and artillery elements? I don't think you really need to diminish their signature here, but you can be creative. Because there are already embeded in the code tests which will protect support (and arties are of this kind) if lines can take the pain... Also, supports having already few hits, compared to lines elements, they have anyway few chances to be picked, unless in a dire situation. But its your call.
This parameter is mostly done for NCP I believe, but perhaps skirmishers and such can use that too.