When you get hired at just about any airline, you will have travel agreements with many OTHER airlines as well. They are two categories. It just depends on the carrier and the agreement between YOUR airline and the OTHER airline. BOTH types of passes are NRSA ONLY. Non-revenue space available seats, not a confirmed seat.

ZED, Zonal Employee Discount. The fare you pay is based on mileage ZONES. I can't remember the specific breakdown, but say any departure and destination combination between 0-599 miles is a flat rate of $35 dollars. The next mileage ZONE being 600-1200, costing $50, 1201-1800 $65, etc.

Note, the zones and prices are just EXAMPLES ONLY.

ID, Inter line, or Inter airline discount is the other type. An ID90 means that it is 90% off of the highest published Y class fare for that route. You pay 10%. Note, it's the HIGHEST fare. There are also ID95's, ID75's, ect. As above, it's what your airline negotiated with the other carrier. You may get an ID90, YOUR parents may be eligible for an ID75.

A ZED is almost always cheaper.

To give you an example, my company has a ZED agreement with QANTAS, and a ID agreement with Air New Zealand. BOTH AZ and QA fly LAX to Auckland non stop. The ZED fare on QA, $215 rountrip. The ID90 on AZ, $550 round trip.

Both types of tickets are refundable within a certain date. You get them from your employee travel office/bureau. You can also buy them from your airline's ticket counter if the agent is savvy enough to pull it up for you. Like the other guy said, you are the LOWEST priority standby passenger.

Say you are using a ID/ZED on United to get somewhere. Their OWN employees, spouses/dependents of employees, companion/buddy pass riders will ALL get a seat before you.