To define a subnotebook as the Joplin recipient notebook, it says "Subnotebooks can be defined via Project\Scans".
I tried "Notebook/Subnotebook" and "Notebook\Subnotebook"; both do not work. I do not understand what "Project\Scans" means; somebody please explain.
I have just had a test of this and the format for the target notebook seems to be notebook name\sub notebook name. It is a backslash even on Linux. From my tests the notebook names can include spaces and do not have to be quoted, they are case sensitive and the notebooks must already exist. You can also go more than one level deep.