I've seen the Cargo Forum has just exploded with posts about FedEx Disputed Pairings. After reading through some of those posts, I couldn't exactly glean what a DP is from them. They seem to be a trip that you can pick up in open time, but there's something about them that made some unwritten rule come to be that it's not cool to pick them up. What causes them to be so disliked by most pilots? Why can't the unions do anything about them? Are there DPs at other airlines besides FedEx?

I'm just finishing up my multi commercial training and am looking to learn as much as I can about all the facets of the industry I hope to be part of here in the near future, so any help is greatly appreciated.