I can understand the xp system. I will explain my views here, though.
I view the promotions system, as "skills", of sorts. Combat I indicates a general proficiency in handling a sword. Combat II is advanced parrying and disarming techniques. Guerilla represents time spent training in rough/hilly environments, and getting used to that terrain. Woodsman represents knowledge of the forests, how to climb trees, what berries are poisonous, etc.
These skills are obviously aqquired through training and/or combat. For the "Veterancy" concept here, I see it as the "feel" of killing a man. Bypassing the mental barrier, and striking the killing blow to an enemy. After having done it once, and gotten over the initial shock, a soldier becomes much less hesitant in the future, raising his overall combat effectiveness. It would also represent seeing firsthand, friends dying around you. Comrades falling on the field of battle.
Generally, I think Veterancy should represent a mentality, that no amount of training in barracks and academies can teach. The XP system doesn't quite cover it, because there are many ways to gain xp without directly fighting, like civics, wonders, buildings, feasting, etc.
