Centre Back
A Centre-back (also known as a CB, central defender or centre-half) is a defender whose preferred position is in the area in front of the goal and is supposed to prevent opposing players (particularly centre forwards / strikers) from scoring.
Centre-backs' jobs are blocking shots, tackling, intercepting passes, contesting headers and marking attackers to prevent them to receive the ball.
In FIFA Ultimate Team, there is no Position Modifier card for changing a card's position to CB or change a CB card's position to other positions.