ZED fares are valid on multiple airlines and routes (in theory) and are priced via zone and type (high, medium, and low). ID90 fares are valid on the airline and citypair that you purchased it for.

Both must be purchased in advance at a ticket counter of the airline you work for (not the airline you wish to fly on), however certain airlines have an e-ticket ZED program that allows you to purchase tickets online. Once you buy the ZED/ID90, you have to list with the carrier you will fly on.