I haven't found anything that explains what this does. It sounds like a minimum for something but I don't know what for. Try putting a number in it and clicking preview, then change the number by about 20 and click preview. See what is different. I have found some forums explaining what to put there and all say from 5-9 so changing it to 20 should leave an obvious result. If it has to do with the two-pass rendering option next to it, it could take a while to create a preview. BE WARNED!