President Trump’s tariffs are changing yet again.

On Friday, the Trump administration imposed new duties on imports from more than 80 countries, including Canada, Mexico and the members of the European Union. Now, many goods arriving from these countries will face tariffs between 10 percent and 12.5 percent.

The new taxes are meant to replace a 10 percent duty that Mr. Trump imposed globally shortly after the Supreme Court invalidated a previous wave of sweeping levies. By law, that replacement tariff could be in place for only 150 days absent congressional approval, and the clock ran out on the president at midnight.

The new tariffs are different from those that Mr. Trump has applied to cars, steel and a variety of other products on national security grounds. The president has also singled out Canada, which will soon face a 50 percent tax on a wide range of products it sends to the United States, as the White House negotiates the future of its trade pact with Canada and Mexico.

Despite the ever-shifting policies, the stakes of Mr. Trump’s tariffs remain unchanged. The president sees high duties as a way to encourage more domestic manufacturing and protect U.S. industries, but taxes on imports tend to fall hardest on American families and businesses. Mr. Trump’s policies may also face a familiar set of legal challenges that could send the administration back to court, where it has lost repeatedly.

The latest: Tariffs targeting unfair trade practices

Mr. Trump’s newest tariffs apply to some of the country’s largest trading partners. To impose the duties, he invoked Section 301 of the Trade Act of 1974.

Under the provision, the president can issue tariffs if the U.S. government finds evidence that other countries engaged in unfair trade practices. The process to investigate those matters can be lengthy, but the Trump administration initiated its inquiries in March, seeing the law as a way to eventually restore some of the rates struck down by the Supreme Court.

One set of investigations targeted dozens of countries and the European Union over concerns that they had failed to take steps to curb “forced labor,” including slave labor and other coercive practices. The rates, which were announced in June and then updated, took effect on Friday.

There are special exceptions and rules for certain products and trading partners, including the European Union. Each country’s rate is determined by the extent to which the administration says it has addressed issues related to forced labor.

A second set of investigations focuses on 16 trading partners, including China, some other Asian countries and the European Union, for what the U.S. government describes as “excess capacity,” or the overproduction of certain goods in manufacturing. This slate of tariff rates is not final yet.

The Trump administration has also singled out Brazil, using the same trade provision to impose a 25 percent tariff on its exports this month. The administration cited a range of unfair trade practices by that country, which Mr. Trump previously sought to penalize over its treatment of Jair Bolsonaro, a political ally and former Brazilian president who was sentenced to prison over a failed plot to hold on to power.

Separately, the Trump administration took direct aim at Canada this month.

On Monday, the president announced that the United States would soon impose a 50 percent tariff on goods arriving from its neighbor, including cement, dairy products, hockey sticks, plywood, paper and wine, alleging that Canada had discriminated against U.S. industry.

To do this, Mr. Trump tapped an obscure provision of another trade law, Section 338 of the Tariff Act of 1930, which permits tariffs as a response to countries that put undue burdens on U.S. imports. The tariffs are set to take effect in August. It is unclear how this duty will interact with those imposed on Canada under Section 301.

Unlike the president’s past actions targeting Canada, this tariff will apply even to goods that are covered under the United States-Canada-Mexico Agreement, known as U.S.M.C.A. That trade deal, which Mr. Trump signed in 2018, is being renegotiated.

Still in place: Tariffs on national security grounds

Dating back to last year, some of Mr. Trump’s tariffs target products regardless of their country of origin. To accomplish this, the president invoked Section 232 of the Trade Expansion Act of 1962, which permits tariffs to address national security concerns.

The president has announced such duties on an evolving set of imports, including steel, aluminum, car parts, heavy-duty trucks, patented drugs, and lumber and wood products, including bathroom and kitchen cabinets and upholstered furniture.

Generally, these duties are not added on top of those that are applied to exports from a particular country.

The tariffs largely have targeted products and industries that Mr. Trump would like to bolster domestically. The rates have also fluctuated considerably, with the White House taking steps to lower the tariffs in response to rising prices or to accomplish other goals.

Now obsolete: The president’s most aggressive duties

Initially, Mr. Trump tried to wage his global trade war using the International Emergency Economic Powers Act, a 1977 law known as IEEPA. No president before him had interpreted the statute to apply such duties. But Mr. Trump tried to wield the law to raise or lower rates with the stroke of a pen, even though such powers are typically reserved for Congress.

After a set of states and small businesses sued, the Supreme Court struck down Mr. Trump’s entire tariff regime in February, forcing the president back to the drawing board. In the process, the government had to repay the money it had collected — more than $160 billion.

Immediately, Mr. Trump turned next to Section 122 of the Trade Act of 1974. That allowed him to impose nearly universal tariffs on all import for up to 150 days, unless Congress agreed to extend them.

Before the time elapsed, however, a federal court again ruled against Mr. Trump, finding that he had not met the narrow criteria under the statute that would allow him to tax imports uniformly. But the court left the tariffs in place as the government commenced its appeal. Lawmakers on Capitol Hill made no effort to preserve the 10 percent rate before it expired in July.