The Kennedy classification system is primarily determined by the most posterior edentulous area in the dental arch. This classification guides the design of removable partial dentures by identifying the type of tooth support available. The extent of the edentulous span and the number of spaces are secondary considerations, as the classification emphasizes the position of the last missing tooth relative to the natural teeth. Proper identification of the most posterior missing tooth is crucial for planning the prosthesis design and ensuring adequate support, retention, and stability.

Reference: Prosthodontics: Principles and Management Strategies, Deepak Nallaswamy, 3rd Edition.