The Trump administration announced its tax plan on Wednesday, calling it the largest corporate tax cut in history. The proposal includes a significant reduction in the corporate tax rate, changes to the standard deduction for individuals, and a one-time repatriation of foreign earnings.

One thing the plan does not include is the much-touted border adjustment tax. President Donald Trump has been off-again and on-again in his support of that proposal, which is one of the key elements in tax reform schemes advanced by House Republicans. Treasury Secretary Steven Mnuchin said a BAT “doesn’t work in its current form” but didn’t rule it out completely.

The border adjustment tax has been the subject of some confusion, as administration officials in the earliest days of the administration described a BAT as a way to get Mexico to pay for a border wall. That rhetoric was quickly exposed as ridiculous and eventually dropped.

A BAT is a key part of the tax reform plan that Speaker Paul Ryan and his cohorts in the House would like to see. It represents a fundamental change in how business income is taxed in the United States.

The BAT comes to raise revenue lost by lowering the corporate tax rate from 35 percent to 15 percent. BAT supporters see the measure as a pivot from taxing production to taxing consumption. As such, they argue, it would encourage exports because revenues generated from goods shipped overseas would be taxed at the new low 15-percent rate, making them more competitive in overseas markets.

BAT proponents also argue that their scheme brings the United States more in line with how business income is taxed in most of the rest of the world in two ways: by lowering the corporate income tax rate and by charging a border tax. While it’s true that many countries have lower corporate income tax rates and do charge a value-added tax (VAT) at the border, the problem is that that is not the whole story.

Countries that impose VATs, charge imports and domestically produced goods at the same rate. They do not discriminate against foreign goods, but the Republicans’ BAT would, because it is imposed only on imports and not on domestically produced products. As such, many experts believe that it would run afoul of World Trade Organization rules and would provoke legal action by US trading partners.

On the other hand, the White House proposal, without a BAT would blow a hole in the federal government’s budget deficit to the tune of $5.5 trillion to $7 trillion, according to a report from the Bureau of National Affairs. So don’t be surprised if a BAT makes its way back into the tax reform discussion. The powers that be just have to make it consistent with WTO mandates.