This is the description from the Scriptorium on the EDU file.

Unit's bonuses against mounted units (if any). There can be bonuses against horses, elephants, chariots and camels.The bonus may be against an entire class of mounts (eg elephants) in which case it's applied versus all of the types of the class or against only a specific type (eg 4amount mumak), but type-bonus stacks with class-bonus. They are applied directly and stack with any other bonuses, like ones derived from spear attributes. Note that these are modifiers to the unit's attack and that they will be applied to a secondary weapon too if one exists. Max number of valid mount effects is 3, extra will be ignored.