JeromeGrasdyke of CA said the following...

What bonuses do weapons have against other weapons, eg swords vs spears, long spears vs other spears

The spear bonusses are fed into the stats system in a couple of different ways, via the mounts bonusses (which are direct attack modifiers against units mounted on that particular mount type) and via the 'spear' weapon attribute. Units which are marked with spear gain a +4 combat bonus against cavalry and use the cavalry's charge bonus against them, while cavalry get a -4 attack penalty against them.
https://forums.totalwar.org/vb/showp...6&postcount=19