Just because two airlines are CASS, it doesn't mean they have jumpseat agreements with each other. A reciprocal agreement is when both airlines agree to take jumpseaters from each other. It's usually brought about by flight management and the pilots at each carrier. Your company or pilot group should provide you with an approved jumpseat list. If your company is not on one, it doesn't mean you can't get a ride but it just makes it harder to get one.

Some airlines require listing prior to jumpseating and some don't. Again, this is something that would be found in your reciprocal agreement. The only thing CASS allows you to do is sit in the cockpit if no seats are open in the back. Just because an airline isn't CASS, doesn't mean they don't offer jumpseats. You might be able to sit in the back.

Hope this helps...