I assume I can use whatever sounds good...
That's basically it! The main goal is to have someone with "fresh ears" or "different ears" take over from the mixing engineer for any
final touches to the sound, and to make sure all of the songs on an album sound good when played together in sequence.
And generally,
you are going to use the same best-sounding master you can make no matter what the distribution format. You
might be able to do some tweaks so that a low-bitrate streaming MP3 sounds a
little better, but it's the job of the compression algorithm to make the best of what you feed it. And, you want to put your effort into the good-sounding stuff, not the stuff that's going to sound like crap anyway.
There may also be format requirements or recommendations. (i.e. CDs should be 44.1kHz/16-bit, and iTunes should be AAC, etc.). But, you really have to check with wherever you are sending it. Most download services will do a conversion if you upload a format that's not standard for them, so it shouldn't be too critical. But, but in any case, you should use a high-quality format (or high bitrate, if compressed) since they may transcode and degrade it.
A CD pressing plant may require a DDP file. The DDP file has
everything the pressing house needs. If you are having CDs burned, an ISO file should be acceptable, and a master CD might even be acceptable.
I have no idea if there are any standards for vinyl. It's a "specialty item" and the pressing house is is likely willing to use whatever format you give them. (Of course, you are going to get more respect if you send them a high-resolution format.) You'd probably have to give them instructions for what goes on each side, and you might have to ask about their time-limit for each side.
There is also a trend in to make your recording "louder than everybody else" by using tons of compression & limiting. (
Loudness War)
"Loudness" is the responsibility of the mastering engineer. Achieving the loudness you find on commercial CDs is difficult for amateurs to do at home without introducing unacceptable amounts of distortion. And, sometimes the pros go-ahead and introduce distortion (clipping).
iTunes Radio is countering the loudness wars by volume-adjusting everything before streaming. So for iTunes Radio, you'd be destroying all of the dynamics for nothing.
A vinyl or high-resolution version may be less dynamically compressed, but that's up to you (or the producer).... There's no
technical reason for using more dynamic compression on a CD or MP3. It's just a matter of marketing and the loudness wars. In fact vinyl has
less dynamic range than CDs or MP3s, but again for marketing reasons you might want to have a different more dynamic master.
I have some good links on mastering -
What's This Mastering Business Anyway? (Moulton Labs).
Tips & Tricks For Mastering (Moulton Labs).
Mastering With Ozone (Izotope).
Mastered For iTunes (Apple).
Details of a particular mastering job (REAPER forum).