Smooth withdrawals are amounts we calculate should be withdrawn from retirement accounts to maintain a consistent withdrawal level in today's dollars from year to year. Smooth withdrawals can be differentiated from required minimum withdrawals (RMD). So where a RMD may be required at age 73, one could set smooth withdrawals to begin at age 80 and this would impose RMD from age 73-79 and then smooth withdrawals would begin at age 80 in an amount calculated to liquidate the account by the age of last withdrawal that you set.
In inherited accounts, RMD may be required in the current year so, for example, if you are 62 and you set your smooth withdrawals to begin at age 68, you will see RMDs from 62-67 and then smooth withdrawals begin at age 68. Depending on the age of the IRA owner's death, you may also be limited to a ten-year window to liquidate the account and forced to take RMD at the minimum for some but not all of the years since the account must be fully liquidated. Thus, you may take smooth withdrawals across the entire set of years, or you may take RMD for a period of years and then liquidate the account with smooth withdrawals for a final set of years or even for the last year alone.
The global defaults for the age of first smooth withdrawal and last smooth withdrawal are set in Settings > Retirement Accounts > Age of First/Last Smooth withdrawal. Each spouse has his or her own setting.