For example, say you are currently using a 4-3-3 formation. The positions are, LB, CB, CB, RB, CM, CM, CM, LW, ST, RW. Take Iniesta for example. On some cards, he is CAM. However, for maximum effect in the 4-3-3 formation, you want Iniesta to become a CM. Thats when you buy the card to switch from CAM to CM to achieve the desired effect. I hope this helps.