I am not a qualified expert in this area, but I think I have an answer.
17 USC is copyright law. My references come from this website:
U.S. Copyright Office - Copyright Law of the United States
17 USC 408 (b) - Copyright registration in general:
...
(b) Deposit for Copyright Registration. — Except as provided by subsection (c), the material deposited for registration shall include —
(1) in the case of an unpublished work, one complete copy or phonorecord;
(2) in the case of a published work, two complete copies or phonorecords of the best edition;
(3) in the case of a work first published outside the United States, one complete copy or phonorecord as so published;
(4) in the case of a contribution to a collective work, one complete copy or phonorecord of the best edition of the collective work.
Copies or phonorecords deposited for the Library of Congress under section 407 may be used to satisfy the deposit provisions of this section, if they are accompanied by the prescribed application and fee, and by any additional identifying material that the Register may, by regulation, require. The Register shall also prescribe regulations establishing requirements under which copies or phonorecords acquired for the Library of Congress under subsection (e) of section 407, otherwise than by deposit, may be used to satisfy the deposit provisions of this section.
So everything hinges on what "phonorecords" means.
17 USC 101 says:
“Phonorecords” are material objects in which sounds, other than those accompanying a motion picture or other audiovisual work, are fixed by any method now known or later developed, and from which the sounds can be perceived, reproduced, or otherwise communicated, either directly or with the aid of a machine or device. The term “phonorecords” includes the material object in which the sounds are first fixed.
Because it says
material object, it seems to me that you should send them one or two ordinary CDs, presuming that you're going to make/sell CDs. If you're doing this 100% electronic (i.e. mp3 downloads, no physical product) then there must be another answer. (FWIW, "copies" is also defined as meaning a "material object").
Interesting question and something good to know. Again, I'm not an expert, I just did a little research and this is what I found.
Edit - upon re-reading the definition of "phonorecord", it sounds like you can send them
format you like on that material product, provided that somehow you can reproduce your music from the "phonorecord". (see: "fixed by any method now known or later developed" and "from which the sounds can be perceived, reproduced, or otherwise communicated, either directly or with the aid of a machine or device"). It is my understanding that the copyright office is not going to review or analyze what you send them - just keep it on file in case there is infringement some day. So they might not care what you send them, so long as you can reproduce the music somehow. Just a guess.
