I don't NEED this. I WANT this. Even without it, Joplin is great. But yes, my left panel of notebooks has notebooks, and subnotebooks and often sub-sub-notebooks. It is very much an outline system. It has personal stuff, research and business stuff all in a single file. I have notes on Sextus Empiricus writings, clippings from news, vendors for home repair, detailed notes on what I've done (project notes) for many clients with notebooks for each client. I even have notes for when certain programs will come out on streaming media.

Under business I have software under that I've got subfolders for various softwares. Joplin has 19 notes.