3097 Willow Ave,
To preserve the tax benefits of a plan, the plan must provide substantive benefits for rank-and-file employees, not just business owners and managers. These requirements are called nondiscrimination rules and compare both plan participation and contributions of rank-and-file employees to owners/managers.
Traditional plans are subject to annual testing to assure that the amount of contributions made on behalf of rank-and-file employees is proportional to contributions made on behalf of owners and managers. In most cases, safe harbor plans are not subject to annual nondiscrimination testing.