Papers in a folder but not belong to a group can not be quickly located.

[YEYE33]

If I search a paper in a folder but not in any group, the column of "Groups" will be blank. It is not conducive to fast find the localization of the paper.
I think the Folders will be meaningless if the Groups can be added under other groups as sub-groups. In other words, a reasonable situation is that Folder are sometimes treated as a Group. In Papers3, there is only one form called "Collection", and Collection can be moved under other Collection as a sub-Collection.
I think it's better if Bookends remove "Folder" and just reserve "Group" which can be sub-Grouped.

[Jon]

A paper can't be in folder. A paper can be in a group, and that group can be in a folder. But a naked paper cannot be in a folder. One note: the Groups column will contain only static group names. Smart groups aren't be listed (it would bog down the UI if there are many to sift through. You can always see which smart groups a reference belongs to on demand, though, by pressing Option-Shift).

I don't understand what you think is a problem and what you're asking for.

Jon
Sonny Software