What are "Unlisted Items"?

It’s automatically in my “Lists” on the left side of the app window. Right know I see 13 of my images in the “Unlisted” designation. Searched for “Unlisted” in User Guide and this Support section, no results.

The special list of ‘unlisted items’ is only useful in combination with your own lists. When you select the project name, you can see all the items in the project. When you select a list only the items you added to that list are visible. The special ‘unlisted items’ will show all items in the project which do not belong to any list.