Never mind. I finally found the distinction in a piping reference "The Piping Guide" by David R. Sherwood and Dennis J. Whistance.
Apparently HALF-COUPLINGS are used for 90 deg branch connections into larger pipe or vessel walls. It is stonger than a Full-Coupling. However, since the HALF-COUPLING will need to be shaped using a SOCKOLET is a better fitting selection for this application.
Contrariwise, the REDUCING INSERT is used to conveniently make a size reduction in the line.
Thanks anyway. I found my own answer merely moments after I posted.