"Steeping efficiency" - which I take to mean the extract obtained when steeping grains.
I don't ever "steep" grains so I have no idea how well that correlates...
[edit] From Beersmith3 Help: "The steeping efficiency is the mash efficiency from steeping grains. Typically only a small percentage (10-15%) of steeped grains end up in the wort after steeping."
@Mario Jammaers (or future readers): if you are brewing with DME/LME and crystal / roasted malts, there is opportunity for further discussion. If you are brewing "all-grain", it's likely that the value of that setting won't matter.