What does boundary force field affect?

So can anyone tell me what does that boundary force field slider affect and how to optimize it for all-star level game?

It affects how prone the players are of stepping out of bounds.
If you have the slider low the players will more easily step out of bounds; and if the slider is high they will have it harder to step out of bounds.
As for an All-Star game, it doesn't really affect the gameplay that way so that it will be more realistic to use a certain setting in an All-Star game, but I guess if I'd have to say anything it would be to higher the slider a very little bit since the game is more relaxed and so on...