On the "Withdrawal Amount" tab you can calculate a matrix of potential periodic withdrawal amounts for a given starting amount (PV) while varying the withdrawal term and interest rate for the annuity (or investment account).
The "Present Value" tab allows you to calculate a matrix of various starting amounts (the present value) that would be required for a given withdrawal amount for different terms (number of withdrawals) and interest rates. In other words, how much do you need to start with to have your investment last for "X" withdrawals at various interest rates.
The "step values" on either tab control by what amounts the interest rate and term are going to increase.
Note: It is not necesssay to clear one calculation before doing the next. You can change one value and recalculate.