I'm only a level 3, but as far as I'm concerned, they're used for how aggressive/conservative a player is during a match. Like, let's say you're playing three at the back and no one at Central Defensive Mid (CDM). I would either put blue arrows on both central midfielders or wing players to cover the space on defense. Or, if you're play style is to play the wings, you would use the red arrows on your Right Mid (RM) and Left Mid (LM) so they'll cross the ball more.