Saichō’s most innovative classification systems were devoted to revealing the differences between the practices advocated by the various Buddhist schools. One of the most important of these systems was based on a hierarchical classification of five types of bodhisattva found in the Pu pi ting ju ting ju yin Ching (Niyatāniyatagatimudrāvatārasūtra). Each type was compared with a man who was attempting to travel to a distant place. If the man boarded the wrong type of vehicle, he would never arrive at his destination because winds and storms would constantly delay him and force him to turn back. If he chose a more suitable vehicle, he would eventually arrive at his destination, but only after a very long period of time. If he rode the best vehicle, he would quickly arrive at his destination. The five types of bodhisattvas described in the sūtra and their vehicles were:

1. 1. The bodhisattva who followed sheep vehicle practices (yōjōgyō)
   2. The bodhisattva who followed elephant vehicle practices (zōjōgyō)
   3. The bodhisattva who followed the practices of the vehicle which endowed him with superhuman powers enabling him to reach the sun and moon (gatsunichi jinzū jōgyō)
   4. The bodhisattva who followed the practices of the vehicle which endowed him with the superhuman powers of a Śrāvaka (shōmon jinzū jōgyō)
   5. The bodhisattva who followed the practices which endowed him with superhuman powers like those of the Buddha (Nyorai jinzū jōgyō)

The sūtra did not describe the actual practices which each vehicle represented. Rather, the main theme of the sūtra was that not all religious practices were of the same efficacy. Some practices enabled a person to advance rapidly towards his religious goal without any danger of backsliding. Others, especially Hinayāna practices, caused him to regress, removing him further away from his goal than ever. Although the sheep vehicle and the elephant vehicle were both subject to backsliding, the other three vehicles were not.