In the Ashtanga Yoga system there are 6 different sequences. They are Primary Series , Intermediate Series and the A,B,C + D Advanced series.
The Primary Series is referred to as Yoga Chikitsa which means Yoga Therapy. The therapeutic aspect of the practice is built through the sequential order of the postures as each one works as a foundation for the next asana in the sequence. Through the purification factor of the practice the body becomes stronger and the skeletal system is realigned.
The Intermediate Series (sometimes referred to as second series) is called Nadi Shodana. Nadi Shodana means nerve cleansing, this is done through the specific postures of this sequence which work to open and clear the subtle energy channels of the body.
Once the students has strengthened the body with the Primary Series and has a good foundation with this sequence only then are they ready to focus on the purification of the nervous system with the Intermediate postures.
The A,B,C & D postures of the advanced series are known as Sthira Bhagah which means steady strength. Higher levels of strength and flexibility are created with the advanced sequences , and only after years of a disciplined practice of the Primary and Intermediate postures would the student have built the right physical and mental foundation to begin the advanced series.
Guruji Sri K Pattabhi Jois is to have said about the different sequences
Primary Series very important, Intermediate is of some importance and the advanced series for demonstration only