Note that the term "elective deferral" refers to money from the employee's pay that the employee instructs the employer to deposit in the traditional 401(k), exclude from box 1 of the employee's W-2 and report with code D in box 12 of the W-2. Money from the employee's pay that the employee instructs the employer to deposit in the Designated Roth Account in the plan are not called elective deferrals but are instead called Roth contributions. (Don't expect TurboTax to always follow this distinction between these terms; there are probably places in TurboTax where it refers to Roth contributions as Roth elective deferrals despite that being incorrect wording.)
The only difference with a contribution to the Designated Roth Account in the 401(k) is that the amount would not be included on Form 1120S line 17 because it is not deductible. It also would not reduce the amount on line 7 for officers or line 8 for other employees. In other words, Roth contributions will not be reflected anywhere on Form 1120S. The only place it appears in tax reporting documents is with code AA in box 12 of the officer's or other employee's W-2.
I did not mean to suggest that Form 1125-E would be required when the entity has less than $500,000 in total receipts. I was using the calculation method on Form 1125-E simply to justify not including elective deferrals on Form 1120S line 7 or 8 and including them on line 17 instead.
