Question: what is after tax 401k contribution?
Answer: Like a Roth 401(k), an after-tax 401(k) contribution is just that, made after taxes are paid. Like a Roth 401(k), earnings grow tax-deferred. However, unlike a Roth 401(k), the earnings on the account are taxed upon withdrawal. The after-tax option predates the Roth 401(k).