“Aum Tat Sat,” this has been considered to be the threefold designation of the Eternal. By that were ordained of old Brâhmanas, Vedas and sacrifices.

The chapter pivots to its closing formula, and each syllable is worth unpacking since none of it survives translation intact. Om (Aum) is the praṇava, the primal invocatory sound used to open Vedic recitation and ritual; Tat (“That”) is the classical Upanishadic way of pointing at Brahman without naming it, keeping the Absolute one grammatical step removed from any specific description; Sat (“Being,” “the real,” “the good”) names what is actually, permanently so. Together the three function as a kind of stamp — a way of consecrating an act to ultimate reality rather than merely performing it correctly.

Study — Śraddhātraya Vibhāga Yoga