I've got two questions now
1. How could I have found that out on my own?
2. How do I find out how the Auto DJ function works?
#1 would be pretty hard to do on your own, except by doing what you did - ask on the forum. I agree that it's not intuitive. You would assume that it has something to do with shuffle mode as it appears with those selections. It is unfortunate that the player bar page on the wiki is still under construction at this time.
For #2, their is some info available on the wiki. Hitting the "?" button when on any the preferences panels or using the "F1" key whenever MB is open, will take you to the MusicBee wiki.
If all else fails,
read the instructions ask on the forum