Required Minimum Distribution Reporting

I have reported my RMD several years using TurboTax, and the amount has always appeared on Line 15b/1040.  This is my wife's first RMD year.  She got an RMD distribution from an employer qualified saving plan (401K).  The amount appeared on Line 16b along with our pension amounts.  The difference seems tp be that 1099's with Box 2b checked as "Taxable amount not determined" end up on Line 15a/1040, whereas the 1099's with Box 2b not checked end up on Line 16b/1040.

Regular persion payments are not part of the RMD.

How does the IRS know that  she took her RMD?  Is the 1040 the vehicle, or are there other systems in place that tell the IRS that the RMD was taken?

We have done everything right to take our RMD's but I just don't understand how our efforts have been reported to the IRS.

Thanks,

Bob [personal info removed]