In the main windows, a group is marked by a special symbol, “–”.
I cannot find it in the documentation: DEVONthink Pro Office 2.9.17 Documentation, page 142, makes no mention of it.
What does it mean?
It’s the common symbol used for a mixed state. Often seen in checkboxes.
It means the contents have more than one state, like some items are flagged and some are not.
Oh. Now that you mention it 