The Hatha Yoga Pradipika names four postures as the essential ones and then goes further, singling out siddhasana from among those four and calling it the chief. The text says that of the eighty four postures Shiva taught, this one should always be practised, that it purifies the channels of the subtle body, and that it opens the door of liberation. Those are Svatmarama's claims and the atlas records them as his.
The description is exact. One heel is pressed firmly against the perineum, the other foot is set above the genitals, the chin is drawn toward the chest and the gaze is held steady between the eyebrows. Every element of that has a function in the system: the heel maintains a pressure the corpus associates with the lowest of the inner locks, the chin position is the throat lock, and the fixed gaze belongs to the concentration practices of the later chapters.
Read alongside the rest of the book, siddhasana is not one option among fifteen. It is the posture the Pradipika's whole apparatus of breath restraint, locks and seals is designed to be performed in, and the reason the text can afford to be brief about the other fourteen. A practitioner who had only this entry would still have the seat the book assumes.