It's entirely dependant on the company you work for. You need to talk to them for the answers to most of your questions. In general, ID90 offers a discounted ticket on a standby basis only. You normally still pay taxes & surcharges. You can use ID90 on any carrier participating in the agreement; this usually means flag carriers, but not LCCs.
ZED is a website that allows you to check likely availability and to list yourself for a flight.