Leadership Ability Derivation

Leadership Ability: This rates the character’s capability to give orders and have them obeyed. Leadership ability determines the number of activation points (AP) that a commander receives. It also determines the number of divisions a general may have in his army. Leadership ability ranges from 1 to 8. 

So I searched for all other strings of Leadership Ability on my PDF version of ACKS Domains at War: Battles. I cannot find what defines what the leadership ability is composed of. I would presume it is some combination of ability and proficiency...or is this one of those rules entirely up to the GM to determine. 

Leadership Ability: A character’s leadership ability is equal to the number of henchmen the character may retain, e.g. 4 plus the Charisma bonus or penalty. If the commander has Leadership profciency, the commander’s leadership ability is increased by 1. The maximum leadership ability is 8.
D@W Battles page 91.