You always need to be a part of a compatibility zone. That is not optional.
A Business Network is something layered on top. It’s like a form of access control.
If we indulge in a nice fantasy, we can imagine one day that every business in the world is part of a single jumbo-sized compatibility zone. Everything from huge banks to small corner shops.
However, that does not mean your application wants to e.g. initiate a complex derivatives swap trade with that tiny corner shop the other side of the world. You probably want to add more sophisticated rules and onboarding, and check your counterparties have signed those agreements, on top of what the compatibility zone already provides. Then you want to add a Business Network.