From Ex illis wiki
Reaction Priority (RP) represents a warrior’s ability to hold his/her ground and strike first when charged. It is compared to the attacker’s Charge Priority to establish the charge order. The difference between the RP and the CP values determines the defender’s chances of performing a Charge Reaction (either before or after being charged). Longer weapons like spears and pikes have the highest Reaction Priority. Certain Defensive Stances and Terrain Features in a zone sometimes increase Reaction Priority.
This value does not increase with Levelling.