The word ātmā refers to the living entity. From Lord Brahmā down to an insignificant ant, everyone is considered a living entity. Living entities are considered part of the Lord's marginal potency. All of them are kṣetrajña, knowers of the body. When they become nirgrantha, or free, saintly persons, they engage in Lord Kṛṣṇa's service. That is the sixty-first meaning of the verse.