The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
3. Beyond lightning (there is) Varuna, on account of the connexion (of the two).
The Khândogya continues, 'From Âditya to the moon, from the moon to lightning.' Here Varuna (mentioned in the Kaushîtaki-upan.) has to be brought in so that above that lightning he goes to the world of Varuna. For there is a connexion between lightning and Varuna; the broad lightnings dance forth from the womb of the clouds with the sound of deep thunder, and then water falls down. And a Brâhmana also says, 'It lightens, it thunders, it will rain' (Kh. Up. VII, 11, 1). But the lord of all water is Varuna, as known from Sruti and Smriti.--And above Varuna there come Indra and Pragâpati, as there is no other place for them, and according to the force of the text, as it stands. Varuna and so on should be inserted at the end, for that reason also that they are merely additional, no particular place being assigned to them. And lightning is the end of the road beginning with light 1.
386:1 So that Varuna and so on are to be placed after lightning.