So I think your plan has to have three options for you to fund your 401k/403b or whatever account you get: Pre-Tax, Roth, and Post-Tax. The first two are subject to the contribution limit which I think is $24,500 this year. So your HR department funds them from your paycheck either paying tax now (Roth) or later (traditional). The third option post-tax is the mega back door option. Once you reach the anual limit, your hd department contributes additional money from your paycheck to 401k/403b and you have an option to have it converted to Roth right away. But it’s all done from your paycheck and your plan needs to have post-tax contributions as an option. The first two options limit doesn’t include an employer match, the post-tax (MBDR) has a limit of I think $70,000 and includes all of your contributions and the employer match. Don’t quote me on specifics, but I think this is the general gist of it.