Defensive matchups allow you specify which positions each player on your team may defend. You can check the box next to each position that you want the player to potentially defend in a game. A player will always be eligible to defend their current offensive position by default. The settings grid shows each player's adjusted defensive rating at each position. This adjustment is based on the player's effectiveness at that position, but is not strictly effectiveness multiplied by defensive rating.
The SimEngine will automatically optimize your team's defensive matchups based on the offensive players, the defensive ratings of your players, and the positions you allow each player to defend. Offensive players are evaluated based on shooting efficiency and number of touches.
NOTE: You may also elect to turn off the matchups, in which case your players would just defend their current offensive position.