If you were to tell the printer to go to (20,20,20) while the printer was in absolute mode, it would result in the printer first moving to (20,20,20). If you told it again to go to (20,20,20) it would just stay there. The Duet would say: I am already there! If you told the printer to move (20,20,20) twice while the printer was in relative mode. The printer would move 20mm in the X, Y and Z directions the first time. And then move 20mm in the X, Y and Z directions for the second time as well. This is the difference between absolute and relative.