A Safe Harbor 401(k) plan generally satisfies the non-discrimination rules for elective deferrals and employer matching contributions. For a 401(k) plan to be considered a Safe Harbor plan, employers must satisfy certain contribution, vesting, and notice requirements. In a Safe Harbor plan matching contributions vest immediately.
So, let's say you have 25 employees, but only a third of them choose to put money into the plan. There's a formula that limits the amount you can personally contribute to a traditional, non-Safe Harbor plan. Under a Safe Harbor 401(k), if you're willing to make a minimum contribution on behalf of your employees, you'll be able to maximize your own personal contributions to the plan.