This is a requirement of the WARN (Worker Adjustment and Retraining Notification) Act. A WARN notification is required if more than 500 people (or 33% of the “active workforce”) are being laid off.

这是美国《WARN法案（员工调整和再培训通知法案）》A部分的规定：在超过500人（或33%的‘有效员工’）将被裁员时，企业必须发布WARN通知。

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The federal WARN (Worker Adjustment and Retraining Notification) Act requires employers to either give a full 60 days notice before closing a plant or engaging in a mass layoff or to pay dislocated workers 60 days of wages and benefits, including health insurance premiums.

FORBES: Layoffs And Lawsuits