Remix Releases 101
Let's say artist A makes a track, and artist B wants to remix it. B will most likely create a remix from just the CD audio of A's track, or from acapella tracks (only the vocal from the original song - often obtained from studio contacts or second hand from the same source, but many hip-hop and electronic acts make acapellas freely available, see the Beastie Boys, or even release them as B-sides, see Eminem) and send it to A's label, perhaps along with a description of what B would do given better access to A's master recording (every instrument as a seperate audio file).
A's label now has three options. It can ignore B's correspondence altogether - the most common option by far. B can then either leave it at that, or can release the remix as an unnamed, unbranded release, which will be termed a White Label or VIP remix. B's name will not appear on the release and B will likely only be recognised as the remixer by rumour and hearsay, as the release is in effect a copyright infringement. It will most likely only appear on vinyl, at pressings of between 100-3000 copies, so intended for DJ play only. A's label can pursue the originator, as the vinyl pressing house's mark will appear on the disc, but as a cost vs benefit thing, they probably wont. If a house DJ plays a house remix of a U2 track at a house club, it lets a room full of people who likely wouldn't listen to U2 hear a U2 track, thus promoting U2 at no expense to the label.
A's label's second option is to release B's 'rough draft' remix as it stands. This happens rarely, but does happen. The remix will most likely be found as the B-side to the 5th single off the album, or on the Japanese release, or as a DVD extra to the tour video, or appears as an 'exlusive' on a A-label-promoted DJ mix album. Point being that it does get released but not so as you'd notice as a consumer. B gets paid some money for this and A owns the copyright to the remix, and everyone's happy.
Third option is that A's label likes where B is going with the remix, and releases the track stems (every individual instrument) to B to finish the remix. B will then finish the remix as planned, but A's label (and often A himself) will have more creative control, as if B was an artist on the label. B will get paid well for this, but at the expense of his creative control. The remix would then get released as a B-side to the original song's single or to the lead or second single off the album. These days, these third option remixes would get put on iTunes Store etc, but the 2nd option remixes probably wouldn't. In the case of A and B both being on seperate independent labels, A's label would release the stem tracks in exchange for someone from B's label being remixed by someone from A's. Then B would get paid less well, but both labels would be cross-promoting each other effectively.
Note that this is when B WANTS to remix A. It is often that A's label thinks that B is *so hot right now* that they will actively pursue a remix from B. Hence last year you get Burial (Mercury award winner, NME and Radio 1 favourite, but with plenty integrity in the underground, exclusively signed to an independent) remixing Bloc Party (on V2, owned by Virgin, difficult indie rock band on their difficult 2nd album) and Thom Yorke (Radiohead frontman on his inaccessible but admittedly brilliant first solo album). This will follow the 3rd model above, but B can pretty much name his price for the remix.
And just to end with my favourite internet cliche, hope that helps!