If you are receiving a pension payment, then ALL of the pension payment applies to the RMD (see ChristinaR's explanation, above).
If you are taking a distribution from an IRA or 401K, some or all of that distribution may constitute a Required Minimum Distribution.
Go to this website for information on How to Calculate Required Minimum Distribution (RMD).
After determining what your RMD is for a given tax year, you can make the required withdrawals. In the case of having multiple retirement accounts, you will have to decide whether to take the entire RMD from one account, or to take partial distributions from several accounts.
If you take more than is needed from any one account (to meet the RMD), then you would indicate that only part of that distribution was for your RMD.
