From the perspective of urban planning, there could be three levels or scales connected with each other in a transport-oriented urban spatial structure. One is a single TOD or other urban area at various levels and scales. The other is TOD in groups, nested clusters, and other urban areas at various levels and scales. The third is an urban region, in which various levels of single TODs, groups, and nested clusters of TODs and other urban areas are at their respective scales and sizes, and various characteristics form functions at different levels and scales.