Guarantees are series of tags you can give to troop in troop editor. They are the way to tell game to ALWAYS give every troop an item type (ranged, helmet, shield, horse, armor, boots, gloves, etc.). If they are not checked it is random if unit gets item from that category (like looters not having armor etc.). From what I experienced you need to start new game for changes, in those tags, to take an effect. However I only changed few of them so putting this mod mid game should still add random crap to bandits equipment, but it wont be exactly what I wanted (All forest bandits will have some helmet and ranged weapon for example).