From the Background: "Rules of origin (ROO) are laws, regulations, and procedures used for ascertaining the 'nationality' of imported products. ROO are important for many reasons, including determining the admissibility of imported goods, assessing duty rates, country of origin marking, applying tariff quotas, enforcing U.S trade laws, establishing eligibility for preferential programs and free-trade agreements (FTAs), and collecting trade statistics. Determining origin is relatively straightforward if all of a product's raw materials and parts are manufactured and assembled in one country. However, in today's global economy, parts of manufactured goods to be assembled into products such as automobiles, computers, or clothing, often come from many countries. This can make determining origin a complex process."
CRS In Focus, IF10754
Congressional Research Service: https://crsreports.congress.gov/