Periodic payments are ones that are received in the form of a regular pension or annuity and are dictated by the terms of the plan or contract. If you instead receive monthly payments of an amount that you yourself chose and can change, they are nonperiodic payments.
If you have basis in after-tax contributions to the plan or investment in the contract, the way the taxable amount of each distribution is determined varies whether or not you are receiving periodic or nonperiodic payments. Normally the payer will report the correct taxable amount in box 2a of the Form 1099-R and you should just tell tell TurboTax that box 2a shows the correct taxable amount. Only if box 2b Taxable amount not determined is marked or you believe that the amount in box 2a is wrong should you consider using the Simplified Method for calculating the taxable amount. (TurboTax only offers to use the Simplified Method for periodic payments, which is why TurboTax asks if the payments were periodic.)
