Cavvy mark are notches left in the mane to indicate the horses level of training.
Snaffle bit, shaved clean no notch.
Two rein, 2 notches about an inch or apart.
Bridle horse, one notch.
This is/was done for a couple of reasons...
If you have your ranch horses turned out on the desert you can tell them apart from a Mustang.
Also if there is a lot of cowboys coming and going the new guy when he gets his string cut to him can tell what he needs to ride his horses in.
In the pictures below, you could walk out and see my black horse is in the bridle because he has one notch.(His needs to be redone, it is a little overgrown) And my bay colt is in a snaffle because he has no notches. I don't have anything in the two rein so I can't show you that.