Sorry, I can't express my problem without having to abstract it into something simple, I hope gets my point across. It has also been pretty much impossible to search Google etc., for this particular issue as I can't properly formulate it.
Imagine a content type called "Order".
You go to create a new Order. Where you can choose a car, and tires for THAT car. The only tires you can choose, have been defined on the car itself, when it was created.
So if you choose a tiny car, the tractor wheels are not shown as a choice of tires. Because on that tiny car, the tractor wheels haven't been selected.
On the car node, there's a field_usable_tires, where you can select the different tires that car can use.
My problem is how to make the "tires" selection on the create Order form change, dependent on the tires the car can have, when a car is selected.
Really hope someone out there understands my issues and can help! :)