Code D only applies to distributions from nonqualified plans. Distributions from traditional and Roth IRAs are from qualified plans and cannot have code D in box 7. A Form 1099-R reporting distributions from an inherited traditional IRA will have only code 4 in box 7 with the IRA/SEP/SIMPLE box marked and a Form 1099-R reporting distributions from an inherited Roth IRA will generally have code T in box 7 but could have code Q instead if the account has been open for at least 5 years.
