I've just checked on my account and I think I understand the process now. I think non-member means someone without a paid membership, so with just a supporter number they get for free.
You need to add that supporter number to your F&F list then you can set them up for Ticket Transfer. So if they have a supporter number you can add them as a non-member.
But City don't make it easy to understand and they never run these communications past City Matters so that ordinary fans can see if they can understand it. I don't wonder why everyone is confused.