The Pro-C can be expanded with up to three more 3-station modules or a 3-station and a 9-station module. Thus, a contractor no longer needs to stock ten different types of controllers (3-, 6-, 9-, 12- and 15-station units in both indoor and outdoor models)…he simply needs to keep on hand just four items (indoor and outdoor Pro-C base units, 3-station modules and 9-station modules).