For most center hung applications, the bottom arm is attached to the closer spindle and the door is resting on top of the arm. This application can be identified by the presence of the door centering screws.
Door centering screws are located on the face of the door near the lower edge where the bottom arm is attached. There should be a large countersunk finished washer between this screw and the door face.
The centering adjustment is more critical for pairs of doors to get the meeting stiles to align when the doors are in the closed position. This is especially the case with DOUBLE ACTING DOORS, single or pairs.