It's the expected standard deviation of income growth, entered as a percentage. If you do not have such a purchase, enter zero for the purchase amount. Enter the amount of savings you will spend on a home purchase, or zero if you won't dip into your savings to buy a home. Enter how many years off that is in this field. Enter the amount of income you will need at retirement.
Nothing is certain in life, especially in a probabilistic model.
There are no guarantees that you'll hit your retirement income target, so specify the level of risk you're willing to accept. Try different confidence levels to see how the recommended investment strategy varies.