How do taxonomists decide which rank to assign to a natural group?

Taxonomists do not rely on explicit, universal rules to define the rank of a natural group, as there are no specific taxonomic characters that dictate whether a group should be classified as a phylum, class, order, or other category. Instead, the assignment of rank is often a matter of scientific consensus and historical precedent within specific fields of study.1

The process generally involves:
* Recognizing Natural Groups: Taxonomists first identify groups based on resemblances to and differences from known forms.1
* Evaluating Relationships: Traditionally, this was based on gross anatomical structures, but modern taxonomy increasingly utilizes phylogenetic (or cladistic) approaches, which group organisms based on evolutionary relationships and shared derived traits.2
* Consensus and Revision: Because there is no objective measure for the "maximal difference" allowed at each level, the rank assigned to a group can vary between authorities. As new information—such as DNA analysis—emerges, classifications are subject to revision.12

Ultimately, while the Linnean hierarchy provides a framework, the specific ranking of a clade is often considered arbitrary, and some modern systems may forgo formal ranks entirely.2