The basic premise is:
go to your carrier ticket desk

buy a zed fare from them to use on contracted afifiiated carrier for specific travel between airport A and B

pay for the zed (based on the mileage, but ticketed city to city within that zone--500 mile, 1500 miles, 2500 etc)space available

show up for that origin/destination with appropriate zone level tkt (Zed L, M or H) with their lsiting requirements and ask to be activated for whichever flight you hope to take
you have already paid the fare--if you don't get on that route, you can refund the ticket at your carrier's tkt desk