There is a difference between withdrawals from after-tax 401(k) plans versus Roth 401(k) plans.
Contributions to both an After-tax 401(k) plan and contributions to a Roth 401(k) plans are made on an after-tax basis. However, there is a big difference when it comes to distributions. When you make qualified withdrawal from an after-tax 401(k) plan, the portion of the distribution that represents your contributions to the plan will be tax free, but the investment gains will be taxed as ordinary income. Conversely, a qualifying withdrawal from a Roth 401(k) is completely tax free.
So if your RMD is from an after-tax 401(k) plan, there will be a taxable amount to the distribution. The amount of the taxable withdrawal will depend on the ratio or your contributions to the plan versus the earnings in your after-tax 401(k) account balance. Your 401(k) administrator should be able to give you information as to what portion of your distribution is considered the taxable portion.
In a 401(k), pre-tax and after-tax contributions are subject to RMDs at age 73. You can avoid the RMDs by rolling the after-tax amounts to a Roth IRA.
@naz1 Thanks for your question!