A standard defensive formation consists of 1 up to 3 central defenders and a left and a right wingback. Central defenders are the heart of your defense and obviously their defending skill is their most prominent attribute.
You cannot field more then 3 central defenders, or you will lose training that week. This means there's a maximum of 10 trainees for defending trainers.
The abbreviation CD is commonly used for central defender.
Normal -- The defending skill contributes to the defence ratings—mainly the central defence, but also a lesser contribution to wing defence. A lone central defender will contribute equally to both wing defence zones, while one of a pair will contribute only to the side on which he is playing. In addition, the playmaking skill contributes a small amount to the midfield rating. Offensive -- The defender uses less defending and more playmaking. His defending still contributes to all three defence ratings. His playmaking contributes to the midfield ratings (though less than an extra inner midfielder or even a winger towards middle Towards wing -- More of his defending contributes to the wing defence (and less to the central defence). Also, he will contribute some wing skill to the wing attack on his side. Afkomstig van Hattrick NL, de Vrije Encyclopedie. "http://wiki.hattrick.org/Central_Defender"