Before revision:  By distinguishing local and global dispersion forces, we can reduce a wide variety of extant models into only three prototypical classes that are qualitatively different in implications .
After revision:  By distinguishing local and global dispersion forces, we can define three prototypical classes, namely, models that have only global, only local, and both local and global dispersion forces. The three model classes engender qualitatively different spatial patterns. Multiple cities are formed only when a global dispersion force is at work; otherwise, only a unimodal distribution can be formed. A city can have its spatial extent only when a local dispersion force is at work. A wide variety of extant models into only three prototypical classes that are qualitatively different in implications .