How do I create a main instance?

Hello,

when I want to create an application component in the process scenario edtior I can choose a "main instance". However how do I create a main instance? I suppose that this has to be done in SLD but I found nothing there.

When can a main instance and a product be different? I do not yet fully understand the concept behind it as both consist of several product component versions that can be installed on a technical system, right?

Any explanation is highly appreciated. Thank you.