Import quotas have returned to fill the void Trump's tariffs created.

US officials have so far largely relied on tariffs to reduce imports of steel, aluminum, and Chinese goods. Even though an import quota may sound nicer than a tariff, its effect on the economy can be just as bad.

Economically, there's little difference - both tariffs and quotas are designed to reduce the quantity of imports and raise prices. However, a tariff at least produces revenue for the US government. A quota, by contrast, instead lets foreign producers keep an equivalent windfall from higher prices.

"Economists generally don't like either of them," says Monica de Bolle, a senior fellow at the Peterson Institute for International Economics, "but if you need to have one, tariffs tend to be better than quotas. Tariffs at least operate through the price mechanism."

South Korea was the first to strike a deal. It will reduce steel shipments 30% below the average level of the last three years.

The Commerce Department said on May 1 that it had reached quota agreements with Argentina and Australia. It also proposed a steel quota for Brazil, but Brazilian officials said they had not agreed to it.

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The European Union is also looking into the option of tolerating quotas on metal imports to the US in an attempt to avoid a trans-Atlantic trade war, according to EU officials. The condition for such a deal would be that any US limits on steel and aluminum from the 28-nation bloc be set at levels no lower than its 2017 shipments to the American market, the officials in Brussels said. EU exports to the US last year of both metals were worth a combined $7.6 billion.

Trump is using the tariffs as leverage in NAFTA negotiations with Canada and Mexico. He's given them until June 1 to agree on quotas.

"Economies are dynamic, they're always evolving, and you have this fixed quota in place," de Bolle says. "The fixed quota could become more and more binding, and the distortion could become ever greater."

EU leaders plan to discuss trade and the US during a dinner session at an informal summit in Sofia, Bulgaria, later this month, according to reports.

Before the initial waiver expired, the Trump administration offered the EU a quota equal to 90% of last year's shipments to the US. The EU refused the proposal. Last week, EU national diplomats established that no limits below the bloc's 2017 export levels to the American market would be acceptable.

The upshot of using quotas is that it is a means by which to avoid a trade war. If the US goes ahead with steel and aluminum tariffs, the EU has threatened to retaliate against products including Levi's jeans, Harley-Davidson motorcycles (HOG  ) and bourbon. The return of quotas will bring in a slew of changes that may wreak havoc on international markets, at least temporarily.