But verily there exists another higher which is un-manifest, different from the other un-manifest [the causal world or Prakriti in its seed form], which is not destroyed even when all beings are destroyed.
The Lord says there is one more avyakta or "un-manifested" state even higher than Prakriti. And that is Brahman, the substratum of this lower Prakriti which is also un-manifest.
Then how is this higher avyakta different from the lower avyakta? Bhagavan uses three words to show the difference-
(1) भावो = it is of the nature of existence, which means the other - the lower Prakriti is only inert. It seems to live, be enlivened only because of its substratum that is of the nature of Existence.
(2) परस्तस्मात्तु = It is Higher than the other avyakta, Prakriti. Higher is not here to mean in geometric fashion, but what it means is it is even the substratum of the cause and effect (Prakriti). Or it means that Prakriti is dependent on Brahman for its existence, so it is called "lower", but the other way is not true, Brahman can remain independent of Prakriti.
(3) सनातनः = It is eternal. Brahman remains unchangeable, imperishable in all the three periods of time. It is never destroyed even when the the other things are destroyed around it, including as the Lord himself said- "all words including Brahman ji" which all fall in the realm of Prakriti only.
Here a question may arise, why the word avyakta (un-manifest) is used both for Prakriti as well as Brahman.
Maya or Prakriti is also un-manifest in its seed form. When we for example go to sleep, all our impressions remain in the seed form , called the "causal body". When we wake up it comes out and manifests. Since they were un-manifest in the middle, they are also called as "avyakta". Similarly in the case of Brahmaji's creation - all these worlds, all our minds and everyone's minds put together go into un-manifest state at Brahmaji's "night". Therefore Prakriti is also called as un-manifest or "avyakta".
Brahman is also called un-manifest for it never becomes an object of perception. Hence really speaking compared to Prakriti, it is Brahman which is truly "avyakta", for Prakriti manifests and then goes back to un-manifest state. But Brahman ever remains as it is, never coming in the realm of duality of cause and effect.