What it is
No federal law requires severance. Employer plans, handbook language, employment contracts, and union agreements create it. The payout format changes the math: a lump sum is taxed in the year paid, salary-continuation payments spread the tax, and structured payouts have their own discount rate.
What to do about it: Ask for the eligibility rule in writing before signing anything: length of service, formula, release requirement, and the treatment of accrued PTO.
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.