"Payroll company, which by definition is in the business of processing payroll, asked my client to confirm tax reporting for after-tax contibutions. To which I responded, 'can you please have your payroll confirm who the HCEs are for 2021'? (Kidding, of course.) My understanding is that as far as tax reporting goes on W-2s and 941s, these deductions are no different than 401k loan payment. Am I correct?

I think it's one of those things where I can't find any articles on how to report it on W-2s and 941s because there's simply no requirement to do so. I'm trying to prove a negative, is the other way to look at it.

I did find this in the W-2 instructions:

'Reported in box 14, but not in box 12. -- After-tax contributions that are not designated Roth contributions, such as voluntary contributions to a pension plan that are deducted from an employee's pay.'

And Box 14 is apparently just a 'whatever you want it to be' box, nothing regulatory about it."