There is an important difference between hierarchical PCT and subsumption.

In subsumption, higher-level controllers operate instead of lower-level controllers. When a higher-level controller needs to do something, it overrides lower-level controllers in order to do it. The robot senses an obstacle, so the "walk forwards" code is suspended while the "avoid obstacle" code takes over driving the legs.

In HPCT, higher-level controllers operate by means of lower-level controllers. When a higher-level controller needs to do something, it does so by setting reference levels for lower-level controllers. When the robot encounters an obstacle, the reference for desired direction of motion is changed, and the walk controllers continue to do their job with a different set of reference signals. The obstacle-avoidance controller does not even need to know whether the robot is on legs or wheels, only that it can send a signal "go in this direction" and it will happen. Each layer of controllers, in effect, implements a set of virtual actuators for the next level up to use.