We automatically calculate how much to withdraw from your accounts to maintain your spending level and use all your assets -- these are called "smooth withdrawals." We also automatically account for the IRS-mandated Required Minimum Distributions (RMDs). In addition to those types of withdrawals, you may want to draw specific amounts from your retirement accounts in specific years to provide liquidity to address a cash constraint, to help pay an expense, or for other reasons. These are called "special withdrawals" and you can enter them under each retirement account. These special withdraws will override a smooth withdraw, not add to it. If the special withdraw is less than the RMD, the RMD will be used instead.
Note that while you can enter special withdrawals under each individual account, the program will use assets from all Roth accounts to satisfy special withdrawals from any Roth account and, likewise, from all non-Roth accounts to satisfy special withdrawals from any non-Roth accounts.
The Special Withdraw option is in any one of the retirement account panels near the Roth Conversion option. See below.