"requirements" probably the only thing I can think of for this one is that you can't wait too long to get listed. Or at least that's what they told me back when the allocation program was different. They've revised how you're listed/when you get it back in 04? Oh, another thing is that you must be treating your CF. If you haven't been doing any treatments, doing IV's, or taking pills that they've prescribed then you're probably not going to get listed, since that shows them that you won't be compliant after tx with the medications, doctors appts., physical therapy, etc.
I know that I had pseudo and MRSA pre-tx, so it depends on what clinic you go to I suppose. I've heard that some don't tx b. cepacia patients.
As for how they decide? Your CF doc should take an overall view of your life/health. If you're progressively declining in PFTs/ quality of life despite the fact that you are taking good care of yourself, they'll most likely refer you to a tx clinic/doctor with a recommendation for an evaluation or a preliminary appt.
bout all I can think of right now, though as Jane suggests go through the tx section, also do search for 'transplant' on this site, should bring up some results.