The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
3. (The rite of carrying fire on the head is an attribute) of the study of the Veda (of the Âtharvanikas); because in the Samâkâra (it is mentioned) as being such. (This also follows) from the general subject-matter, and the limitation (of the rite to the Âtharvanikas) is analogous to that of the libations,
With reference to the pûrvapakshin's averment that the rite of carrying fire on the head is connected with the vidyâ of the followers of the Atharva-veda only, not with any other vidyâ, and that thereby the vidyâ of the Âtharvanikas is separated from all other vidyâs, the following remarks have to be made.--The rite of carrying fire on the head is an attribute not of the vidyâ, but merely of the study of the Veda on the part of the Âtharvanikas. This we infer from the circumstance that the Âtharvanikas, in the book called 'Samâkâra' which treats of Vedic observances, record the above rite also as being of such a nature, i.e. as constituting an attribute of the study of the Veda. At the close of the Upanishad moreover we have the following sentence, 'A
man who has not performed the rites does not read this;' here we conclude from the word 'this' which refers to the subject previously treated, and from the fact of 'reading' being mentioned, that the rite is an attribute of the study of the Upanishad of the Âtharvanikas (but has nothing to do with the Upanishad itself).--But what about the immediately preceding passage, 'Let a man tell this science of Brahman to those only by whom the rite of carrying fire on the head has been performed according to rule?' Here the rite in question is connected with the science of Brahman, and as all science of Brahman is one only, it follows that the rite has to be connected with all science of Brahman!--Not so, we reply; for in the above passage also the word 'this' refers back to what forms the subject of the antecedent part of the Upanishad, and that subject is constituted by the science of Brahman only in so far as depending on a particular book (viz. the Mundaka-Upanishad); hence the rite also is connected with that particular book only.--The Sutra adds another illustrative instance in the words 'and as in the case of the libations there is limitation of that.' As the seven libations--from the saurya libation up to the sataudana libation--since they are not connected with the triad of fires taught in the other Vedas, but only with the one fire which is taught in the Atharvan, are thereby enjoined exclusively on the followers of the Atharvan; so the rite of carrying fire on the head also is limited to the study of that particular Veda with which scriptural statements connect it.--The doctrine of the unity of the vidyâs thus remains unshaken.