His mastery extends right to the ultimate atom and to the ultimate magnitude
When he concentrates on it, he can steady his mind on anything subtle, right down to the ultimate atom; when he concentrates on it, he has steadiness of mind on anything substantial, up to the ultimate magnitude. When one can take his practice to either at will, it is full mastery; when he has full mastery, he does not require further practice in training.
The words right to are to be taken with both the extremes. When he concentrates on something subtle, in the course of his practice the mind experiences things progressively smaller and smaller till he comes to the ultimate atom. By practice he becomes able to remain steady in that experience.
When he can take his practice to either limit at will, it is full mastery. He has complete mastery who is not obstructed by any opposing thought in his experience of either the very small or the very great. The earlier practices are (part of) the highest (mastery), but there is this distinction: when he has full mastery he does not require further practice in training, whereas those in the early stages do require some more training.
There is, then, a three-fold concentration (dhāraṇā): the contracted which touches the limit of minuteness, the extended which touches the limit of greatness, and the third which experiences both of the limits. The sūtra implies all three.
Now when the mind has attained steadiness (sthiti), what sort of samādhi does it have, and on what objects?
When the mind has attained steadiness by one or more of the methods given what sort of samādhi does it have and on what objects? The following sūtra has been given to answer this question.