1- They’re used prioritize animations to what should be played, to give an example: I have 2 examples first one is less priority and second is high priority. While first animation is playing if I play second animation, first animation will stop and second animation will play.
3- It really depends on context and status, example giving: If you like to do a slash animation for a sword tool then you must set it to Action priority, if you would like to re-make the default Idle animation then Idle priority must be used.