This triple designation.

 Every time a necessity arises for the indication of Brahman, these three words, OM TAT SAT are utilized either together or severally. Brahman projects himself as sound for the purpose of manifesting Himself as the phenomenal universe. The sum total of the sound of the cosmos is OM. It is otherwise called Nada Brahman. This sacred sound is interminable. Brahman is referred to by the utterance of OM. He is also indicated by the word TAT which means It. A respectable person is is not referred to by his name but with the simple word he. In such deference, Brahman is indicated with the word "It". Again SAT means the Reality, which remains unaffected by time, space and causation. Brahman is also referred to ad SAT. This triple designation therefore always indicates Brahman.

Though both butter and buttermilk come from the same substance, milk, we only say that butter comes from milk and make no mention of the other substance. Though all things and beings have come out from Brahman, the most prominent productions among them are the Brahmanas, the vedas and the yajnas. The most evolved and the most spiritually inclined ones among men are the Brahmanas. Among the branches of knowledge, the sublimest are contained in the Vedas. And among the activities  going on in the world, the most elevating and beneficial ones are the Yagnas. In the Nada Brahman these three occupy the foremost place

Chapter 17 verse 23.