Field Type "Nestedset" in KnowledgeBase

I had a user ask why they couldn't filter the Category Field in the Knowledge Base.  It resulted in me looking into it, and I've discovered things I don't understand.  As I get older, I've found there's been an alarming increase in the number of things I know very little about :-)

Indeed, there is no capability to enable a search on the Category field.  The Category field is set to a field type of "Nestedset", and there's no ability to create a field with that type and my search hasn't yielded any insight.

When creating a new KB record, the Category field is a drop-down, but I can't find which drop-down list is being used, although I must have created the list in 2012 when I implemented Sugar...

Will someone please point me to where I can learn about the Nestedset Field type, and perhaps how I may find the drop-down list the Category field uses?