Per Champ @Opus17 :
"You do not get a tax deduction now, but the plan administrator will keep track of your basis in the account and this will reduce the amount of tax in the future.
For example, if 10% of your contributions were made after tax, then 10% of your withdrawals when you retire will be exempt from tax. You can confirm this by calling the account custodian."
When you begin taking distributions from your retirement plan, your Form 1099-R will reflect a non-taxable amount, relevant to the after-tax contribution amount.
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