Which coverage test are after-tax contributions tested under?

After-tax contributions in a 401(k) plan are primarily tested under the 401(m) coverage test. This test specifically addresses the contributions made to a retirement plan, focusing on the match and after-tax contributions that are made on behalf of employees. The 401(m) test ensures that the contributions do not disproportionately favor highly compensated employees over non-highly compensated employees, thereby maintaining compliance with non-discrimination requirements.

The nature of the 401(m) test allows for an evaluation of the contributions made in relation to overall employee compensation, which is essential for maintaining the plan's qualified status. While options like 401(a) and 401(k) also deal with qualifications and participation, they do not specifically address after-tax contributions, making the 401(m) the relevant test in this context. The 403(b) option pertains to different types of retirement plans and is not applicable here.