When entering information about a back door Roth IRA conversion from a non-deductible Traditional IRA, you do not need to enter any kind of explanation. The forms and data speak for themselves.
If you are in a section of your return asking for a reason for 'recharacterization', then you have gotten on the wrong track. Recharacterization is when you change your mind and attempt to reverse a conversion.
Take a look at the following TurboTax article for more information about conversions and recharacterizations and the steps you need to take in TurboTax to report a back door Roth IRA conversion.
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