Jim Stone examines base position for defense: In other words, where the diggers should be positioned before an attack happens. This is not always a particular spot on the floor - base position is not always a static position, but instead depends on the opposing team's strengths and movements.
The back row players should generally divide up court responsibilities into thirds. To help visualize this, it might help to draw general circles of responsibility around your diggers. In this particular scenario, the right and left back row diggers cover the middle of the court and to their left and right. The middle digger covers the middle back, and deep attacks to the left and right corners of the court. Base position is flexible and depends on your defensive system and the opponent.