Having told Arjuna what cannot happen to the soul — it cannot be cut, burned, wetted, or dried — Krishna now names what the soul is. Eternal. All-pervading. Firm. Unmoving. Sanatana — a word that carries the weight of 'existing since before the beginning of time.' Nine attributes, each one ruling out a different kind of change or destruction.
Two words stand out. 'Sarvagata' — all-pervading, present everywhere — means the soul is not locked inside one body the way a bird is locked in a cage. It exists everywhere, at all times. And 'sthanu' — immovable, like a pillar rooted deep in the earth — means it does not waver, does not shift, does not respond to external force.
This shloka is the summary verse. Everything Krishna has said about the soul from 2.12 onward is compressed here into a single line. Uncuttable, unburnable, unwettable, undryable, eternal, all-pervading, firm, unmoving, and everlasting. There is nothing left to add. The case is closed.