Tag icon meaning

Can someone help me with the meanings of the icons I have below. This is on DTTG and my tag view where some have a label icon, some have what appear to be groups icon, and one tag even shows what appears to be a small screenshot of a pdf file that is tagged.

Long-press the database on DEVONthink To Go’s databases screen and choose Verify & Repair. Then check the tags groups again. Do things appear normally?

All fixed except the one that looks like the thumbnail of a pdf.

What is the reason it showed as groups (and doesn’t on DT).