The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
55. But the (meditations) connected with members (of sacrificial acts are) not (restricted) to (particular) Sâkhâs, according to the Veda (to which they belong).
The above occasional discussion being terminated, we return to the discussion of the matter in hand.--We meet in the different Sâkhâs of each Veda with injunctions of vidyâs connected with certain members of sacrificial acts, such as the udgîtha and the like. Cp. e.g. 'Let a man meditate on the syllable Om (as) the udgîtha' (Kh. Up. I, 1, 1); 'Let a man meditate on the fivefold Sâman as the five worlds' (Kh. Up. II, 2, I); 'People say: "Hymns, hymns!" the hymn is truly this earth' (Ait. Âr. II, 1, 2, 1); 'The piled up fire-altar truly is this world' (Sat. Brâ. X, 5, 4, 1). A doubt here arises whether the vidyâs are enjoined with reference to the udgîtha and so on as belonging to a certain Sâkhâ only or as belonging to all Sâkhâs. The doubt is raised on the supposition that the udgîtha and so on differ in the different Sâkhâs because the accents, &c. differ.
Here the pûrvapakshin maintains that the vidyâs are enjoined only with reference to the udgîtha and so on which belong to the particular Sâkhâ (to which the vidyâ belongs).--Why?--On account of proximity. For as such general injunctions as 'Let a man meditate on the udgîtha' are in need of a specification, and as this need is satisfied by the specifications given in the same Sâkhâ which stand in immediate proximity, there is no reason for passing over that Sâkhâ and having recourse to specifications enjoined
in other Sâkhâs. Hence the vidyâs are to be held apart, according to the Sâkhâs to which they belong.
To this the Sûtra replies 'but those connected with members,' &c.--The word 'but' discards the primâ facie view. The meditations are not restricted to their own Sâkhâs according to the Veda to which they belong, but are valid for all Sâkhâs.--Why?--Because the direct statements of the texts about the udgîtha and so on enounce no specification. For to such general injunctions as 'Let a man meditate on the udgîtha'--which say nothing about specifications--violence would be done, if on the ground of proximity we restricted them to something special belonging to its own Sâkhâ, and that would be objectionable because direct statement has greater weight than proximity. There is, on the other hand, no reason why the vidyâ should not be of general reference. We therefore conclude that, although the Sâkhâs differ as to accents and the like, the vidyâs mentioned refer to the udgîtha and so on belonging to all Sâkhâs, because the text speaks only of the udgîtha and so on in general.