When a player commits a minor or major penalty, he must serve the penalty himself. However, if a player receives a 10 minute game misconduct (ejection), or a 5 minute major and a 10 minute misconduct, another player is required to serve the penalty for the ejected player, or the 10 minute misconduct while the other player serves the 5 minute major.
In this case, NHL rules allow the coach to specify any player he wishes to serve the penalty. The player you specify in your Advanced Defense settings will serve these penalties, provided he is not already serving a penalty, injured, or ejected. If he is not available, your Sim Coach will select the weakest (most likely to be injured) player on your roster to serve the penalty.