The U.S. decision to launch a slew of new investigations into key trading partners has raised eyebrows among analysts, who question both the timing and objectives of the probes.

The investigations, targeting China, Mexico, the EU and more than a dozen other economies, are being carried out under Section 301 of the Trade Act of 1974.

Here’s CNBC’s brief guide to Section 301s — what they are, why the White House has resorted to using them, and what President Donald Trump’s administration hopes to achieve.

Put simply, Section 301 of the Trade Act of 1974 enables the investigation of perceived unfair trading practices to determine whether “the acts, policies, or practices of a foreign country are unreasonable or discriminatory and burden or restrict U.S. commerce.”

The Office of the United States Trade Representative’s (USTR) Jamieson Greer announced a series of new investigations on Wednesday targeting 16 trading partners, ranging from Singapore and Switzerland, to India and Norway. A full list is here.