When you select Auto, you can offset the sensitivity positivly or negativly. By doing so, you can gain more depth. +1, +2, or +3 should be used in light to moderate mineralized soil. -1, -2, or -3 for highly mineralized soil. By using the Auto sensitivity option, you are allowing the processor to constantly adjust to the ground you are hunting. Basically like using the auto tracking in the Xterra 705. The processor is much better at selecting the sensitivity than most users, but that doesnt mean that you cant run the sensitivity as high as the machine can handle without falsing in manual. Try them both to see which works best for you. I have found in my low to mild soil, I can run Auto +3 all day long without any problems.
I hope that helps.