Okay I think (hope) that I understand now.
I think the outer diameter is 12mm, then the diameter at the edges (where it steps down) is 9mm, and I’m assuming (hoping) it has a 5mm diameter hole through the middle for a standard QR.
The step down bit goes in the drop out and is 9mm wide.
That would make sense looking at the photo, and the standard QR just clamps this axle in place.