Corner kick SA means this player expert in taking the corner kick. you need to set him as Corner kick taker and he will take the corner kick for your team. If you're not set him as corner kick taker, then random player in your team will take the corner kick. Normally i found that corner kick taker can produce good amount of assist not even from the corner. The assist can be from the counter attack and also the cross from the by-line. Last, to answer your question, CKS is the player who expert in taking the corner kick, not the one who heading towards goal... if u are looking for a good header for a cross from corner kick, i would suggest u a defender with Aerial Defender SA.