Plasti-Bond ST style knockout hubs are used to terminate conduit runs through the wall of a sheet metal electrical enclosure. They are available with or without grounding lug. STTB style hubs are Through-Bulkhead" fittings used to pass conduit runs through firewalls or bulkheads up to 1/8" thick. STTTB style hubs perform the same function for unlimited wall thicknesses, (furnished without nipples)."