The Internal Revenue Service is trying to add more details to the rules governing 401(k) plans and other retirement plans that use automatic contribution arrangements.

The IRS has included the additional advice in a notice of proposed rulemaking.

The notice deals with topics such as the safe harbor rules that would apply to “qualified automatic contribution arrangement” and the procedures employers should follow when workers in QACAs eventually decide to opt out of the arrangements.

Employers can choose whether to let workers get unwilling contributions out of 401(k) plans.

Employees must include any employee contributions they end up withdrawing in taxable income, IRS officials note.

Employers will have to forfeit any rejected matching contributions and leave the unwanted matching contributions in the retirement plans, officials write.

The forfeited matching contributions would “be treated in the same manner under the plan terms as any other forfeiture under the plan,” officials write.

A copy of the proposed regulation is available