The Kennedy-Applegate classification primarily categorizes partially edentulous arches according to the number, location, and distribution of edentulous areas, not their length. It focuses on which teeth are missing and where the gaps occur relative to existing teeth. The classification helps in designing partial dentures by identifying support and retention zones. Length or size of edentulous spaces is not a criterion, as it does not significantly influence the classification's purpose. Presence of third molars is also a consideration in subclassification, but length is excluded.

Reference: Contemporary Fixed Prosthodontics, Rosenstiel, 5th Edition.