Why can one scientist classify a group as an order while another classifies it as a class?

The classification of a group as a specific rank, such as an order or a class, is not governed by explicit, universal rules that define those ranks. Instead, the assignment of rank is a matter of professional judgment and widely varying opinion among taxonomists.1

Several factors contribute to these differences in classification:

• Lack of Standardized Definitions: There are no objective taxonomic characters that define a phylum, class, or order. A feature that serves as a diagnostic trait for a class in one group may vary significantly within an order or family in another group.1
• Subjective Interpretation: Taxonomists must first recognize natural groups and then decide which rank best reflects their relationships. Because there is no quantitative measure for the "maximal difference" allowed at each level, one authority may classify a group as a superorder while another designates it as a class.1
• Evolving Methodologies: Different approaches to taxonomy—such as phenetics, which emphasizes overall similarity, and cladistics, which focuses on evolutionary descent—can lead to different interpretations of how groups should be organized.23
• Historical Context: Many established classifications are based on long-standing traditions and consensus within specific fields, and these can be subject to revision as new information from paleontology, genetics, and comparative anatomy becomes available.145