Pass support specifies how much do you want your passes to home in on teammates.
On full pass support, the system assumes the player you want to pass to and no matter how much power you put into it, it always goes to the feet of the assumed player.
On 1, 2 and 3 the degree varies and at some point the power you put into the pass becomes a part of the equation as well. I think, it goes like this
-full automatic - 4
-some of power and direction are considered - 3
-power is considered completely not the direction - 2
-power and direction both are considered, the AI adds some targeting - 1
-Full manual - 0