Speed?
The fourth on the list and an important level as it signified the level just before a change in the portion of the body to cultivate on would take place, which was the Body strengthening was separated into Low, Mid, and High stages for a cultivator to go through before the focus of cultivation would shift from the body and into the bones of the Cultivator.
From then on, cultivation shifts and takes a new turn which would lead to the fifth level of cultivation which is known as the Bone refinement level which was further divided into Low, Mid, and High stages for a cultivator to pass through before moving on to the next level.
The sixth on the list would be the Bone strengthening level which was further divided into Low, Mid and High stages for a cultivator to go through before breaking through and heading for the next level on the list.