The WS-X3004 is a single-port expansion module with a special connector that is installed into the rear panel of the Catalyst 3000. If you have two Catalyst 3000 units each equipped with a WS-X3004 module, then when you connect the modules, the two Catalyst 3000 units function as a Stack. If you have more than two Catalyst 3000 units each equipped with a WS-X3004 module, you can combine them into a unified Catalyst Stack by connecting each module to a Catalyst Matrix unit.