In addition to using them on the same city pair but a different airline (i.e. using a BA ZED LHR-JFK to fly on AA LHR-JFK), there should also be no problem using them on similar flights that should have similar tax, generally flights departing the same city at least, in many cases anything departing the particular country (i.e. using a BA ZED LHR-JFK to board AA LHR-BOS). These scenarios should both work as long as your ZED level (high/medium/low) is at least as high as the airline you are flying and the distance zone you paid for is at least as far as the flight you are taking.

The endorsement on the ticket is something along the lines of "Valid on all ZED-L carriers" or something like that, where the L indicates low zed. Whether they'll be accepted on a different route is up to the agent at the operating carrier, but I suspect most are sympathetic to the plight of a stuck nonrev!

Because interline tickets are issued by the employee's airline, rather than the operating carrier, you can only get them where your airline has a ticket counter. This makes ZED tickets much more convenient than ID tickets, because with the IDs you had to buy a ticket for any possible backup plan you wanted, whereas now you can buy one ZED and use it to cover a lot of backup options. For example, when I went to PRG last year, I bought one ZED ticket that I could use to get to pretty much any USAirways gateway city, on any number of different airlines. It was at a fare and zone level that would have covered PRG-MUC/FRA/CDG/LGW/BCN/MAD/AMS/etc. on OK/LH/AF/BA/IB/KL/etc. Fortunately, I was able to get on OK PRG-JFK nonstop, so I just refunded the backup ZED