a name in three parts, i.e., an infraspecific name (a name for a taxon below the rank of species), needs a "connecting term" to indicate rank. in the "calystegia" example above, this is "subsp." (for subspecies). in botany there are many ranks below that of species (in zoology there is only one such rank, subspecies, so that this "connecting term" is unnecessary there). a name of a "subdivision of a genus" also needs a connecting term (in the "acacia" example above, this is "subg.", subgenus). the connecting term is not part of the name itself.