What it is
Cliff schedules pay 0% before the cliff date and 100% on it, usually at three years of service. Under federal minimum standards, cliff schedules must not run past five years, and plan documents can be faster. A resignation two weeks before the cliff forfeits the whole employer match.
What to do about it: Confirm the service start date, not the hire date on the badge. Clock changes, transfers, and acquisitions reset service credit more often than the plan says.
Plain-language description of US practice, not legal, tax, or financial advice. The paired guide carries the citations, dates, and thresholds; the state pages carry the local windows.