This applies to 401K, 403B or other ERISA plan.
Pretax 401K is no tax now, but tax on withdrawal for both contribution and earning. This is what we usually call 401(k), the limit is 19500.
ROTH 401K is tax contribution now, but no tax on earning when withdrawal.
The Pretax/ROTH 401K has a combined limit of 19500.
Aftertax 401K is contribution is taxed now, and its earning is taxed on withdrawal. It is just like your other after tax savings. The only advantage I can see is that it is inside a ERISA protected account so usually lawsuit cannot get to it.
The after-tax 401K contribution limit is ($38,500). That is ERISA total limit of $58,000 minus Pretax limit of $19, 500.
The contribution part of the after-tax 401K can be converted to a ROTH account. This is called mega ROTH backdoor. The earning part of the after-tax 401K can be rolled over to a traditional IRA.
see https://www.irs.gov/retirement-plans/rollovers-of-after-tax-contributions-in-retirement-plans
Hope this helps.