From the Background: "Section 301 of the Trade Act of 1974 (19 U.S.C. §2411) grants the U.S. Trade Representative (USTR) a range of responsibilities and authorities to investigate and take action to enforce U.S. rights under trade agreements and respond to certain foreign trade practices. Prior to the Trump Administration and since the conclusion of the Uruguay Round of multilateral trade negotiations in 1995, which established the World Trade Organization (WTO), the United States has used Section 301 authorities primarily to build cases and pursue dispute settlement at the WTO. However, President Trump has been more willing to act unilaterally under these authorities to promote what the Administration considers to be 'free,' 'fair,' and 'reciprocal' trade. The Trump Administration's use of Section 301 has been the subject of congressional and broader international debate."
CRS In Focus, IF11346
Congressional Research Service: https://crsreports.congress.gov/