Looks like it’s created when I do an email import, but what does it signify, and what is special about it?
It’s a thread group and will be created for you if you enable the corresponding preference in the Email preferences of DEVONthink Pro Office. It groups related email correspondence together.
Outside of looking pretty, does it have any other special significance, either to the user or DEVONthink or the import process?
I noticed, by default it looks like Classification and Tagging are turned off?
It’s just an organizing tool that some people appreciate (by default it’s off so you much have switched it on explicitly). Since they don’t offer any real advantages for classification or tagging since they’re all considered to be about an identical subject they are set up as such.