What type of bonus does an armor piercing infantry receive against an armored foe?

Lets say, v0 jannisary heavy infantry vs. abyssinian guard (defense 0, armor 1) as compared to vs. ChivFootKnights (defense 6, armor 5).

Jannisary heavy has attack of 5, plus armor piercing. I dont have the slightest idea how to calculated the df for jannisary heavies.

df = attack - defense + bonuses.

All other things being equal, what is the df for jannisary heavies v. abys guard and v. chivfootknights?

Does the bonus increase as the armor of the opponent increases? Is it a one time bonus regardless of the level of armature? That doesn't make sense because even peasants have armor (1). Is there a threshold before the bonus applies (3 armor?). I don't understand this armor piercing bonus.

I'm sure Puzz3d (Yuuki&#33 can help