There are two sets of decision variables in this model. The VOL attribute, defined on the ARCS set, represents to shipment volume from the plants to the customers along each arc. The OPEN attribute, defined on the PLANTS set, is used to represent the plants that are open. Specifically, OPEN( p) is 1 if plant p is opened, else it is 0. The members of the OPEN attribute are set to being binary using the expression:

! Make OPEN binary(0/1);

   @FOR( PLANTS: @BIN( OPEN));