The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
2. But the subtle (body is meant by the term avyakta) on account of its capability (of being so designated).
It has been asserted, under the preceding Sûtra, that the term 'the Undeveloped' signifies, on account of the general
subject-matter and because the body only remains, the body and not the pradhâna of the Sânkhyas.--But here the following doubt arises: How can the word 'undeveloped' appropriately denote the body which, as a gross and clearly appearing thing, should rather be called vyakta, i.e. that which is developed or manifested?
To this doubt the Sûtra replies that what the term avyakta denotes is the subtle causal body. Anything subtle may be spoken of as Undeveloped. The gross body indeed cannot directly be termed 'undeveloped,' but the subtle parts of the elements from which the gross body originates may be called so, and that the term denoting the causal substance is applied to the effect also is a matter of common occurrence; compare, for instance, the phrase 'mix the Soma with cows, i.e. milk' (Rig-veda. S. IX, 46, 4). Another scriptural passage also--'now all this was then undeveloped' (Bri. Up. I, 4, 7)--shows that this, i.e. this developed world with its distinction of names and forms, is capable of being termed undeveloped in so far as in a former condition it was in a merely seminal or potential state, devoid of the later evolved distinctions of name and form.