Something that is not mentioned in the Yoga Sutras that I think is important to know for in depth yoga practice is the four functions of the mind. They coexist and operate on different levels, sometimes simultaneously. They are the Manas, Ahamkara, Bhuddi and Chitta.
The Manas is the lower mind that is responsible for the five input and output mechanisms of the senses. The input mechanisms are sight, hearing, feeling, tasting and touching. Output from the Manas comes in the form of eliminating, reproducing, moving, grasping and speaking. This level of the mind operates on a reflexive level. For example, if you touch a hot stove I hope you do not have to think about removing your hand away from it. Keep in mind that all of our senses and reactions run through the Manas, regardless of the process in between
Chitta is the storehouse of memories and our impressions of them. Samskaras are triggered from the Chitta by input from the Manas.
Ahamkara means "I-maker" in Sanskrit. The closest term I can think of in psychology is Ego, but it's not exactly the same. This is the false identity that we have of ourselves as perceiving the mind as part of matter. For example, there is a sixteen year old boy who just came into possession of a 1992 Mazda 626. The boy, excited by his new car identifies himself with the speed an thrill of his new found freedom, races down Seventh-sixth street and gets pulled over by the Franklin police. "Sir, the far right lane is not the 'racing' lane." says the cop. The boy goes home with a stack of tickets and a broken ahamkara. So the association of himself with the car
himself caused suffering.
The Bhudhi is the higher mind. It decides and judges from a place of wisdom detached from matter. We all have to make difficult decisions in our life. Some happen on a day to day basis, like what to eat, and some have larger implications. The Bhudhi is the part of the mind that decides to buy the sports car when you have the money, instead of going into disastrous debt. Dark Helmet in Spaceball fell pray to the Ahamkara when ordering Spaceball One to ludicrous speed, when the Bhudhi would have just went to the desert planet, you know, where Yogurt lived.
So thought categorization is meant to help us balance these four factors of the mind so that it will become still, allowing use to dig deeper to the next level. A good barometer for discerning the difference between the Ahamakara and the Bhudhi are the types of thoughts and actions that cause use the most suffering. Start with the action or Manas part of the mind, then pay attention to what thoughts may have triggered that action. Actions are always triggered by thoughts and thoughts are always triggered by external stimulus. Symbolism attached to the external stimulus is what churns up the thought associations in the Chitta. When the higher, discerning part of the mind is in control, we can avoid racing into ludicrous actions and bashing our egos.