Ids are based on the price of a ticket for that flight. Zeds are based on block levels of distance and are much more convenient to use IMO. An example of how you could use zeds: if you want to travel from A to B I would buy a zed ticket on every airline that flys from A to B that has an agreement with your airline. The zeds will all cost the same amount because they are based on distance and they are not based on a specific flt number, day, or time. Day of travel you take your zed ticket to the ticket counter of airline you wish to travel on and they list you and print a boarding pass (this is online now some places). If you don't get on do the same for option 2 etc. When you your trip is over return all the zeds you didn't use or flights you didn't make it on for a full refund. Id prices are all over the board and when you are in a foreign country trying to make last minute changes on the fly its a pain in my opinion. Zed equals awesome.