The U.S. Power of Attorney is a legal document that is required by the U.S. Customs and Border Protection (CBP) agency. In order for a customs broker to conduct customs business such as Filing an Entry Summary in your name, they must have a current Power of Attorney on file.
By granting a Power of Attorney, the grantor authorizes the broker to transact with the CBP in their name. Power of Attorney is valid indefinitely until it has been revoked. An exception to this rule is partnerships - a PoA issued by a partnership is valid for two years only, after which time it must be renewed. A PoA can be revoked by a letter of revocation.
For a PoA to be valid, it must be notarized. Corporations with a corporate seal are exempt from this requirement - they can use the corporate certification on the second page of the Power of Attorney instead.
For the POA guide press here.
You can read more about the CBP Power of Attorney here.