गोविन्द भाष्य (श्रील बलदेव विद्याभूषण - अचिन्त्यभेदाभेद वेदान्त ब्रह्मसूत्र भाष्य सटीक)

"Sitting in a yoga posture, and rapt in meditation, the sages gazed at the
Supreme Personality of Godhead."
In this way they who desire to meditate on the Lord are described. Therefore
one should adopt the āsana posture. Otherwise meditation is not possible.
Sūtra 8
dhyānāc ca
dhyānāt—because of meditation; ca—also.
Also because of meditation.
Purport by Śrīla Baladeva Vidyābhūṣaṇa
Meditation is defined as thinking of one thing only and not thinking of
anything else. This kind of thinking is not possible when one is reclining or in any
posture but the yoga āsana. Therefore one should sit in the yoga āsana.
Sūtra 9
acalatvaṁ cāpekṣya
acalatvam—stillness; ca—and; apekṣya—in relation to.
Also because it is related to stillness.
Purport by Śrīla Baladeva Vidyābhūṣaṇa
The word "ca" (also) is used here for emphasis. In the Chāndogya Upaniṣad the
word "dhyāna" (meditation) is used as a synonym of "stillness". There it is said
(Chāndogya Upaniṣad 7.6.1):
dhyāyatīva pṛthivī