I went back and looked at the MA instructions for Sched X & Y so I could try to get a feel for the full context. I think I now get it. That section seems to apply to an IRA or Keogh plan in which you have made non-deductible contributions over a period of years. Let's say you have $50K in the plan and your non-deductible contributions totaled $25K. That $25K has already been taxed. Let's say you've previously withdrawn $15K and this year you withdraw $5K. Since you can withdraw a total of $25K of your non-deductible contributions and only previously withdrew $15K, then the $5K withdrawn this year is non-taxable. So in that context, it makes sense to keep track of all the non-deductible contributions made and all the funds withdrawn.
It doesn't make a lot of sense in the Backdoor Roth situation where every year you make a non-deductible contribution (i.e., funds already taxed in MA), then convert those funds to Roth. I will continue to leave the "Total Distributions Previously Received" blank and hope this doesn't trigger an audit. If it does, it seems like it should be easily explainable.
