SxT=This bulkhead is threaded on the side that is flanged (where you screw in a strainer) and slip on the back side (where you can glue in a PVC pipe).
SxS=This bulkhead is slip both the front and back. A PVC pipe (or other slip PVC fittings) can be glued into either end of the bulkhead.
TxT=This bulkhead is threaded both the front and back. A MPT threaded fitting can be screwed in to either end of the bulkhead.