Lost among President-elect Donald Trump's many campaign promises — the wall and mass deportations, for example — was one about a tax holiday for repatriation.

United States law says that international corporations don't need to pay federal taxes on profits earned through their overseas operations until they bring the earnings back to the United States. Hence, some of them don't — leaving huge amounts of American-held money sitting in foreign banks. Goldman Sachs estimated that as much as $200 billion could be brought back to the U.S. just by S&P 500 companies taking advantage of a one-time tax cut on repatriation.

Will this money go toward fulfilling the president-elect's promises to bring jobs back to America? Will it even come back at all?

Here's a primer on the extraordinarily complicated subject of tax holidays, and what exactly Trump is trying to do here:

How much money is held by U.S. companies overseas?

A lot. Bloomberg estimates that as much as $2.6 trillion is held by American companies overseas. But even this is a guess, as there's little government data to go on, and companies aren't required to reveal the details of their assets and what countries they're held in.

By far, the top offshore hoarder of cash is Apple, holding between $210 and $220 billion. And of publicly-traded companies, Microsoft, Cisco, Google, and Oracle round out the top five, according to Forbes.

Why don't they bring it back home?

Because they'll be hit with the federal corporate tax rate of 35 percent, which is the highest in the world, according to a piece in MarketWatch published in August of this year.

What exactly was Donald Trump's proposal?

As far back as August 2015, Trump spoke of the need to bring corporate profits back to the United States. But his initial proposal would have been even more generous — letting them come back tax free, with the logic that they'd "take that money then and use it for other things." Trump eventually settled on a blanket reduction of the corporate tax rate to 15 percent and a one-time repatriation holiday rate of 10 percent.

Is he the first person to propose this?

The idea of some kind of incentive for companies to repatriate money actually has bipartisan support. Sens. Rand Paul (R-Ky.) and Barbara Boxer (D-Calif.) jointly proposed one in 2015, with the 6.5 percent tax rate they put forth funding infrastructure projects, but it died in committee. Earlier in 2015, the Obama Administration proposed a 14 percent holiday for repatriation, but it went nowhere. And Hillary Clinton floated a repatriation holiday during her presidential run.

Has the U.S. ever done it? And what happened?

The Bush administration passed a repatriation holiday in 2004, a one time cut of the corporate tax to 5 percent, with the provision that the money had to be used for hiring or research, and not executive compensation or buying back their own stock.

None of that actually happened. A report to Congress in 2011 claimed that the holiday cost the U.S. $3.3 billion, and that most of the money brought back was indeed used for executive compensation and stock buybacks. And far from creating jobs, the companies who repatriated money also had mass layoffs. Pfizer brought home $35.5 billion — and cut nearly 12,000 jobs in the next three years. And since then, these companies have stashed even more money offshore.

Will Trump's tax holiday have a different result?

According to Goldman Sachs, it likely won't. "We expect tax reform legislation under the Trump administration will encourage [S&P 500] firms to repatriate $200 billion of overseas cash next year," wrote David Kostin, Goldman's chief U.S. equity strategist, in a November 18 note to investors. "A significant portion of returning funds will be directed to buybacks based on the pattern of the tax holiday in 2004."

"Goldman Sachs is projecting that S&P 500 companies will bring back $200 billion of the $1 trillion in cash they hold outside the United States and use $150 billion for share buybacks," Reuters reported.