The description for the Archer attribute “Bow Master” currently reads:
[Both hands bow] to the public type monster when mounted I will give a 20% additional damage of physical attack.
It sounds like it gives a flat attack bonus (20% sounds too good to be true), but only when using a mounted unit. Archer class, however, isn’t a mount-riding class, and the attribute name lends nothing to using mounts at all. You could read
“to the public type monster when mounted” as a strange way of saying “the effect activates when used on a 'public type monster”, but I still have no idea what that is.