I have no clue what ZED fares are. BUT, as far as ID-90s go, here's how it works. ID stands for Industry Discount and the number following ID-XX is the percentage off the HIGHEST PUBLISHED Y-CLASS FARE, or F-CLASS fare. There are ID-50s, ID-75s, ID-90s, and yes, even ID-95s. Depending on if you base your ID-90 on a Y or F class, you can be upgraded. (Y subject to gate agents attitude, F automatically upgraded, pending availability.)
EX. UA Y26 fare for MSP-DEN is $1000.00. An ID-90 would cost $100.00 base fare plus all applicable taxes and fees. An ID-75 would cost $250.00 base fare plus all applicable taxes and fees. And an ID-50 would cost $500.00 base fare plus all applicable taxes and fees. (** This is a fictional fare, and used for example purposes only. I have no connection to UA, nor do I profess knowlege of their fare structures or pricing.)
As always ID-90s are space-available travel only, but since based on F or Y class fares, they are fully-refundable if not used.
This discount is seldom used with so many corporate and counter-to-counter agreements in place with airlines, but it's sure saved my hide when I got stuck somewhere and didn't have a pass to get me home.
I'm going to Washington, D. C. this week on TZ, and will carry my backup passes on US Airways and Skyway (Midwest Connect) should my flights fill up. If all 3 options failed, then I'd call TZ corporate travel and book my ID-90 on NW.
Unfortunately, depending on airline policy, the boarding priority for ID-90s is pretty low and bottom-of-the-barrel. Thus another reason they are seldom used.
Though I have always wanted to ID-90 an F class fare JFK-SVO on DL, but the idea of getting stuck in SVO is more than daunting.
Hope this info. helps.