So first off, I know close to nothing about the military history of Burma, so please excuse my ignorance if this question has an obvious answer! In the Kingdom of Pagan list it has a unit called "Replace militia spearmen" Which over regular militia spearmen are protected, lose short spear, and gain devastating charger. I didn't see anything in the troop notes about these guys, what do they represent?

The Pagan empire could field large armies, but it was only via the kyundaw method of conscription. I would suggest that the militia listed here are alternative representations of the local tribes who would submit troops to that conscription.