The blue arrow on any defender (DC, DR, or DL) makes them a sweeper. They stay back closer to the goal behind your defense line & go after opponents who get past your defense line; within their zone. For a DC, the zone is all 3 center positions (DC-left, DC-middle, & DC-right). For a DL I believe their zone is only the DC-left; but it might also include DC-center. Vise-versa for DR. If you put blue arrows on a DC-left, that players zone is DC-left & DC-middle. Against an opponent who uses only 1 striker & no AMC, or 1 striker with advanced flanks (AML/AMR) & no AMC, a good tactic is 3DCs with blue arrows on the DC-left & DC-right (gives double sweeper coverage to the DC-middle & normal sweeper coverage to the DC-left & DC-right). DO NOT USE "OFFSIDE TRAP" WITH ANY SWEEPERS; sweepers stand behind the offside line & so you can never catch your opponent offsides with a sweeper.