Each category is referred to as a unit of classification or rank and is commonly called a taxon.

Hierarchy of categories is the classification of organisms in a definite sequence of categories (taxonomic categories) in descending order, starting from the kingdom and reaching up to species or an ascending order from species to kingdom.

The number of similar characters of categories decreases from lowest rank (species) to highest rank (kingdom). 

The hierarchy includes seven obligate categories from top to bottom are kingdom, division or phylum, class, order, family, genus and species.