The purpose of a formal name is to have a single name that is accepted and used worldwide for a particular plant or plant group. For example, the botanical name Bellis perennis denotes a plant species which is native to most of the countries of Europe and the Middle East, where it has accumulated various names in many languages. Later, the plant was introduced worldwide, bringing it into contact with more languages. English names for this plant species include: daisy, English daisy, and lawn daisy. The cultivar Bellis perennis 'Aucubifolia' is a golden-variegated horticultural selection of this species.
The botanical name itself is fixed by a type, which is a particular specimen (or in some cases a group of specimens) of an organism to which the scientific name is formally attached. In other words, a type is an example that serves to anchor or centralize the defining features of that particular taxon.
The usefulness of botanical names is limited by the fact that taxonomic groups are not fixed in size; a taxon may have a varying circumscription, depending on the taxonomic system, thus, the group that a particular botanical name refers to can be quite small according to some people and quite big according to others. For example, the traditional view of the family Malvaceae has been expanded in some modern approaches to include what were formerly considered to be several closely related families. Some botanical names refer to groups that are very stable (for example Equisetaceae, Magnoliaceae) while for other names a careful check is needed to see which circumscription is being used (for example Fabaceae, Amygdaloideae, Taraxacum officinale).
Depending on rank, botanical names may be in one part (genus and above), two parts (various situations below the rank of genus) or three parts (below the rank of species). The names of cultivated plants are not necessarily similar to the botanical names, since they may instead involve "unambiguous common names" of species or genera. Cultivated plant names may also have an extra component, bringing a maximum of four parts:
A botanical 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 not used in zoology). 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.