What are Rules of Origin?

What are Rules of Origin? 

A set of legal standards used in determining what country of origin a shipper can claim as the source of their goods. Different countries, organizations, and trade agreements/associations have different sets of these rules and they are used to enforce tariffs.

Example: 

The USMCA, a trade agreement between the United States, Mexico, and Canada, states in their “rules of origin,” that to be considered a product of a certain country goods must be entirely produced and assembled in one of the three countries to be considered of USMCA origin.