See Income Tax Regulations section 1.401(k)-3(d)(3). Both the traditional and safe harbor plans are for employers of any size and can be combined with other retirement plans.
See also Mid-year Amendments to Safe Harbor 401(k) Plans and Notices.
Roth deferrals are included in the employee's taxable income in the year of the deferral.
Two of the tax advantages of sponsoring a 401(k) plan are: There are several types of 401(k) plans available to employers - traditional 401(k) plans, safe harbor 401(k) plans and SIMPLE 401(k) plans. For tax-favored status, a plan must be operated in accordance with the applicable rules.
The SIMPLE 401(k) plan was created so that small businesses could have an effective, cost-efficient way to offer retirement benefits to their employees.
A SIMPLE 401(k) plan is not subject to the annual nondiscrimination tests that apply to traditional 401(k) plans.
As mentioned earlier, employer matching contributions may be subject to annual tests to determine if nondiscrimination requirements are met.