Nipples are made up with a flange from the same family on each end of a tube section. (Fittings that are manufactured with different flange families on each end are called hybrid adapters.)
Straight nipples are manufactured with the same size flange on each end of straight section of tubing. Reducer nipples have different size flanges (from the same family) on each end.
The three piece unions have to be used in hazardous areas, for the junction between conduits pipes and boxes or various appliances.
The unions are made up of three independent pieces that can be screwed up by rotating the same pieces among them.