ZED (Zonal Employee Discount) fares are based on network zones. This means that there are buckets for 0 to 500 mile flights, 501 to 1000, 1001 to 3000, etc ect. Within each zone bucket there are low, medium, and high fares. What you pay depends entirely on what your company negotiated with the other airline. For example with one company you may get a ZED low for you and your spouse but your parents use a ZED medium. For another company you at get a ZED medium and your parents have to use a ZED high and can only travel if you are traveling as well. Some companies will have business class ZED deals where instead of the normal ZED low, you can purchase a business class option using the ZED high price. You will roll back to economy if there are no business class seats but you don't get the fare difference refunded.
ID90s (and ID75s and IDxx) are an interline discount of XX% on the highest walk up economy fare. They tend to be more expensive and are being phased out across the industry. Other than that, they are the same as ZED fares.
In the bad old days you'd have to go to your airline's employee pass travel office and request a ID90/ZED fare for whatever airline(s) you wanted to travel on. In general ID90s were good for a specific route, and the ZED fare was good for a certain number of miles and could be used on multiple routes. They'd reach out to whatever airline you wanted to travel on that they had deals with (via teletype or something equally as ancient) and get authorization numbers. This could take a week or so. Then you'd get a paper ticket that you'd hand to the other airline you wanted to travel on when you got there, and they'd in turn issue you a stand by ticket. I remember going to Europe once with about 6 different ID90s to use in various combinations to get around. In the end, I only needed to use 2 of them and refunded the other 4 when I got home.
Now, (mostly) you can book your ZED/ID90 online as they are all electronic tickets. Each company is set up a bit different, and you may need to use one or more different websites to book different airlines. We have 2. One covers our internal non rev plus a few other domestic carriers, and then the other covers all of our remaining (114 at last count) ZED fare deals. Many are outdated though, as we still have deals with AirTran and Midwest Express in the system. Some of these portals allow you to create "guest" accounts for other people in your non rev profile. Some don't. Most company's have a policy against sharing your log in, but it's not uncommon to do so. You won't see specific flight loads for any carrier except your own, so you often times have to make educated guesses on how a flight looks based on a smiley face (happy/neutral/sad) or percentage of seats open system. These numbers can be wildly inaccurate, and it's not at all unusual to check before going to sleep and see a green smiley face, only to wake up to head to the airport and see a red frowning face.
Apps like StaffTraveler have made it easier as you can now post a load request and somebody at that company (who has access to their internal non rev system) will, in exchange for future use credits, look up the loads for you. This is borderline illegal in most company's policy manuals, but it doesn't seem to be a problem so far.
Like somebody else said, these seats are all standby. Your boarding priority is normally absolutely last (behind anybody travel standby who works for or is on buddy passes from that company). Your priority within the ZED fare world is normally based on time of check in, although some of the alliances give priority to fellow members. When I was part of the USAirways system, we got ZED priority on other *A member airlines. South African Airlines (no longer flying) had a deal with many carriers that you could buy confirmed (I think round trip) seats from JFK or IAD for like $600. There are also a few other carriers that have/had deals like that, but it's rare.
I've gotten to the point now, where I will buy tickets anywhere that we are going to be time restricted and then non rev when we have more time to spare. For example, we are going to Iceland with my Dad and his wife in October. We have a 4 day window to nonrev/zed fare from home to BOS, but I bought tickets for my wife and I from Boston to KEF because we have to get there the same time my dad does to start out drive. Coming home though, we are going to play the non rev game because we have options on pretty much any flight back to the States and then home. Sometimes it works out perfectly, and then some times you end up having to buy a last minute confirmed seat because it all falls apart.
