The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
24. Vaisvânara (is the highest Lord) on account of the distinction qualifying the common terms (Vaisvânara and Self).
(In Kh. Up. V, 11 ff.) a discussion begins with the words, 'What is our Self, what is Brahman?' and is carried on in the passage, 'You know at present that Vaisvânara Self, tell us that;' after that it is declared with reference to Heaven, sun, air, ether, water, and earth, that they are connected with the qualities of having good light, &c., and, in order to disparage devout meditation on them singly, that they stand to the Vaisvânara in the relation of being his head, &c., merely; and then finally (V, 18) it is said, 'But he who meditates on the Vaisvânara Self as measured by a span, as abhivimâna 1, he eats food in all worlds, in all beings, in all Selfs. Of that Vaisvânara Self the head is Sutegas (having good light), the eye Visvarûpa (multiform), the breath Prithagvartman (moving in various courses), the trunk Bahula (full), the bladder Rayi (wealth), the feet the earth, the chest the altar, the hairs the grass on the altar, the heart the Gârhapatya fire, the mind the Anvâhârya fire, the mouth the Âhavanîya fire.'--Here the doubt arises whether by the term 'Vaisvânara' we have to understand the gastric fire, or the elemental fire, or the divinity presiding over the latter, or the embodied soul, or the highest Lord.--But what, it may be asked, gives rise to this doubt?--The circumstance, we reply, of 'Vaisvânara' being employed as a common term for the gastric fire, the elemental fire, and the divinity of the latter, while 'Self' is a term applying to the embodied soul as well as to the highest Lord. Hence the doubt arises which meaning of the term is to be accepted and which to be set aside.
Which, then, is the alternative to be embraced?--Vaisvânara, the pûrvapakshin maintains, is the gastric fire, because we meet, in some passages, with the term used in
that special sense; so, for instance (Bri. Up. V, 9), 'Agni Vaisvânara is the fire within man by which the food that is eaten is cooked.'--Or else the term may denote fire in general, as we see it used in that sense also; so, for instance (Rig-veda Samh. X, 88, 12), 'For the whole world the gods have made the Agni Vaisvânara a sign of the days.' Or, in the third place, the word may denote that divinity whose body is fire. For passages in which the term has that sense are likewise met with; compare, for instance, Rig-veda Samh. I, 98, 1, 'May we be in the favour of Vaisvânara; for he is the king of the beings, giving pleasure, of ready grace;' this and similar passages properly applying to a divinity endowed with power and similar qualities. Perhaps it will be urged against the preceding explanations, that, as the word Vaisvânara is used in co-ordination with the term 'Self,' and as the term 'Self' alone is used in the introductory passage ('What is our Self, what is Brahman?'), Vaisvânara has to be understood in a modified sense, so as to be in harmony with the term Self. Well, then, the pûrvapakshin rejoins, let us suppose that Vaisvânara is the embodied Self which, as being an enjoyer, is in close vicinity to the Vaisvânara fire, 1 (i.e. the fire within the body,) and with which the qualification expressed by the term, 'Measured by a span,' well agrees, since it is restricted by its limiting condition (viz. the body and so on).--In any case it is evident that the term Vaisvânara does not denote the highest Lord.
To this we make the following reply.--The word Vaisvânara denotes the highest Self, on account of the distinction qualifying the two general terms.--Although the term 'Self,' as well as the term 'Vaisvânara,' has various meanings--the latter term denoting three beings while the former denotes two--yet we observe a distinction from which we conclude that both terms can here denote the highest Lord only; viz. in the passage, 'Of that Vaisvânara Self the head is Sutegas,' &c. For it is clear that that passage refers to the highest Lord in so far as he is distinguished by having heaven, and so on, for his head and limbs, and in so far as
he has entered into a different state (viz. into the state of being the Self of the threefold world); represents him, in fact, for the purpose of meditation, as the internal Self of everything. As such the absolute Self may be represented, because it is the cause of everything; for as the cause virtually contains all the states belonging to its effects, the heavenly world, and so on, may be spoken of as the members of the highest Self.--Moreover, the result which Scripture declares to abide in all worlds--viz. in the passage, 'He eats food in all worlds, in all beings, in all Selfs'--is possible only if we take the term Vaisvânara to denote the highest Self.--The same remark applies to the declaration that all the sins are burned of him who has that knowledge, 'Thus all his sins are burned,' &c. (Kh. Up. V, 24, 3).--Moreover, we meet at the beginning of the chapter with the words 'Self' and 'Brahman;' viz. in the passage, 'What is our Self, what is Brahman?' Now these are marks of Brahman, and indicate the highest Lord only. Hence he only can be meant by the term Vaisvânara.
143:1 About which term see later on.
144:1 Sârîre lakshanayâ vaisvânarasabdopapattim âha tasyeti. Ân. Gi.