Steadfastness in sacrifice, in austerity and in charity is called ‘Sat’. And also the action done for that is called ‘Sat’ only.

Expansion of the use of sat. Sthitiḥ — stability, permanent firmness in yajña, tapas, dāna.

Action tad-arthīyam — done for that (for sat), dedicated to Brahman. It is sat in itself because it is offered, dedicated. Action transformed by dedication becomes real, true.