There is a help article on this, I believe but I'm pretty sure it equates to:
1. Original Gravity. Taken from runnings, pre-boil, pre-pitching... anything prior to fermentation.
2. Anything between OG and finished beer. I use this for checking final gravity readings or anything close to the end of fermentation. FWIW, between my refractometer, hydrometer and BS Refractometer tool, the tool performs the conversion dead on with my equipment. I can check FG with my refractometer only and know that the tool in the software will perform the conversion accurately.
3. Use this to find the starting gravity of a finished beer. If you've forgotten to take the OG of your own brew or you're working out a clone of a commercial product. Using this pull down option with your refractometer AND hydrometer reading will give you an OG for the beer.