I think they come from the MusicBee3Settings.ini file (<TagMoodFilter><TagOccasionFilter>), but thinking about it, that might just be the list for creating filters.
Even if this is where they come from, I don't know if messing with them will mess something up. I don't think so, but backing it up before you screw with it might be a good idea.
You're probably best off waiting for a definitive answer. I edited this post to highlight my uncertainty.