I was wondering what the minimum focus distance switch does. Here's what it looks like:

I thought it was a mechanical lock to prevent you from focusing closer, but had no idea why. When I got to play with a lens that had this switch, I could still focus to minimum distance manually! Then I tried using AF and figured out what it's for:

The switch tells the camera auto-focus not to focus closer than what is selected. Helps the focusing algorithm work faster by eliminating some guesswork. That's it!