Bhagavad Gita, Chapter 17, Verse 27

Steadiness in Yajna, austerity and gift is also called Sat: as also action in connection with these, or, action for the sake of the Lord, is called Sat.

The same word is extended here into something practical, real steadiness in sacrifice, austerity, and gift is also called Sat, not just the abstract idea of truth.

Krishna ties this word to sustained, ongoing effort as much as to eternal reality, action done for the Lord's sake earns the same designation. Real consistency in practice, not just a philosophical claim, is shown counting as Sat too.