In Bhagavad Gita Chapter 17, Verse 23, the speaker (Kaal Brahm) reveals the specific designation of the Supreme God. This verse acts as the "Master Key" to the hierarchy of the Three Purushas that we have tracked through Chapters 7 and 8.
ॐ, तत्, सत्, इति, निर्देशः, ब्रह्मणः, त्रिविधः, स्मृृतः,
ब्राह्मणाः, तेन, वेदाः, च, यज्ञाः, च, विहिताः, पुरा।।23।।
(ॐ) ओं मन्त्र ब्रह्म का (तत्) तत् यह सांकेतिक मंत्र परब्रह्म का (सत्) सत् यह सांकेतिक मन्त्र पूर्णब्रह्म का है (इति) ऐसे यह (त्रिविधः) तीन प्रकार के (ब्रह्मणः) पूर्ण परमात्मा के नाम सुमरण का (निर्देशः) आदेश (स्मृतः) कहा है (च) और (पुरा) सृृष्टिके आदिकालमें (ब्राह्मणाः) विद्वानों ने (तेन) उसी (वेदाः) तत्वज्ञान के आधार से वेद (च) तथा (यज्ञाः) यज्ञादि (विहिताः) रचे। उसी आधार से साधना करते थे।
Om Tat Sat -
‘Om’ is the mantra of Brahm, ‘Tat’ is the coded (symbolic) mantra of Parabrahm, and ‘Sat’ is the coded (symbolic) mantra of Purna Brahm (the Complete Supreme God). In this way, the chanting of these three types of names of the Complete Supreme God has been ordained. At the beginning of creation, scholars created the Vedas and Yagyas (sacrifices), etc., based on that very true spiritual knowledge (Tattvgyan), and they used to perform spiritual practice on that same basis. (23)
This is not a single name, but a bridge that connects the three different "Unmanifest" (Avyakt) powers.
You will notice that "TAT" and "SAT" are not the literal sounds of the mantras. It is clarified that:
This verse proves that the "One-God" theory of common religion is incomplete. If there were only one God and one realm, a single mantra would suffice. The requirement of Om, Tat, and Sat confirms:
According to the subsequent verses (17.24–27):
No. These are "directions." The actual "Tat" and "Sat" are secret mantras that a Tattvadarshi Saint provides during initiation (Naam Diksha). Simply repeating the words "Tat" and "Sat" is like saying the word "Money" instead of having actual currency.
The Gita itself says this knowledge is "Most Confidential." Most scholars stop at "Om" because they only have access to the four Vedas. The secrets of "Tat" and "Sat" are part of the Fifth Veda (Suksham Veda) delivered by the Kavim (KavirDev).
It refers to the Supreme God (Purna Brahm). The verse says that the universe was created through these three names, proving that the entire creation is "encoded" with the vibration of KavirDev.