There are certain things you can do with your income that you can't do with the RMD portion of a retirement withdrawal. For example, you can't rollover your RMD to a Roth IRA. If you withdrew $6000 and your RMD was $5000, you have the option of converting up to $1000 to a Roth IRA but not more. I can't remember the full list of thing you can't do, but if you really want to know, I think some of the other regulars know.
@dmertz
@Hal_Al
In any case, Turbotax asks for the amount of your RMD so it knows that a certain portion of your income is not eligible to be used for these prohibited purposes. It enters the figure on its internal worksheets, so the information will be available later if you indicate you did one of those things that is limited by the RMD rules.
