The plan cost at annuity start date is your total after tax contributions in the plan (may be shown in box 9b of the 1099-R). If you did not make any after tax contributions, leave the entry blank.
If the employee did not make any after-tax contributions to the plan, then the plan cost will be $0 and the entire distribution will be taxable income. If there was any after-tax contributions made to the plan, then a portion of every distribution will be non-taxable.
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