In scientific classification used in biology, the order (Latin: ordo) is
What does and does not belong to each order is determined by a taxonomist. Similarly for the question if a particular order should be recognized at all. Often there is no exact agreement, with different taxonomists each taking a different position. There are no hard rules that a taxonomist needs to follow in describing or recognizing an order. Some taxa are accepted almost universally, while others are recognised only rarely.
For some groups of organisms, consistent suffixes are used to denote that the rank is an order. The Latin suffix -(i)formes meaning "having the form of" is used for the scientific name of orders of birds and fishes, but not for those of mammals and invertebrates. The suffix -ales is for the name of orders of vascular plants.
For some clades, a number of additional classifications are sometimes used, although not all of these are officially recognised.
In their 1998 classification of mammals, McKena and Beel used two extra levels between Superorders and Order: "Grandorder" and "Mirorder".