Bulldøg :
Basically every player has recommended settings in each field. Small example... Attack speed: recommended is 0-30% that means that your player will run slightly slower on attack and play will be pretty accurate (not missing passes and so forth). If you set the attack speed to 100% it will show you that little triangle indicating that you have moved above the recommended setting (players will run alot faster but at the price of accuracy). Hope this helps you a little.
Just to add something. The higher a players points are in recieving, passing, control the higher you can push the "speed" up without worrying about handling errors.