I'm trying to figure out a couple of things about custom and system order types.
1) Is there a way to attach a system order type to a custom order type in the admin UI or it can only be done via API?
2) Where/when exactly system order types are used? Both as a separate entity or as a part of a custom order type.
The thing I'm trying to understand is what are the reasons for attaching system order types to custom order types. What would I be (or not be) able to do in case I (don't) attach them?