I noticed you can only use AXII sign when mounted and only on the horse. Why? Does it reduce fear level or something?
When you are mounted on the horse, the AXII sign is used to calm it down (reduce its fear level).
When used in combat and not mounted on the horse, it is used to stun people.
It can also be used in certain conversations to influence a person (be warned: this does not always yield a favourable outcome).