These couplings are similar to the standard couplings, but are made with an isophthalic polyester resin. This resin is for areas requiring lower levels of corrosion resistance such as water or brine service. This is a less expensive resin.
The stepped coupling is specifically designed for the tank fabricator that wants a half coupling with female threads on one end, for glassing into the side wall of a fiberglass tank. This coupling, in conjunction with a properly sized hole, will stop with half of the coupling projecting from the sidewall due to the step in the coupling.