Series rating applies when you are installing a downsteam breaker that has a lower interrupting rating than the available fault current at that point on the circuit. It is the only way you can make that type of installation.
I really can't accept the implied requirement that every panel and disconnect be of the same make as the main switchgear.
In general a series rating is only obtained by testing. Manufacturer A does not have their breakers tested in combination with other brands. This restriction only applies when you are using a series rating.
Don
