(CNN) President Donald Trump threw Twitter, and members of Congress, into a tizzy Thursday morning when he tweeted a back-and-forth with himself about Section 702 of the Foreign Intelligence Surveillance Act. The reauthorization for 702 was up for a vote in the House just hours later -- it passed, and now goes to the Senate for consideration -- but the President's tweets arguably brought a little understood surveillance tool to the forefront of public discourse.

At 7:33 a.m., @realDonaldTrump tweeted this: "'House votes on controversial FISA ACT today.' This is the act that may have been used, with the help of the discredited and phony Dossier, to so badly surveil and abuse the Trump Campaign by the previous administration and others?"

That tweet seemed to cast doubt on whether the President believed the controversial program should be reauthorized, despite the fact that, hours earlier, it appeared to have the votes. GOP leaders had spent weeks rounding up support and combating demands for changes from conservatives and libertarian lawmakers. The President also seemed to ignore the fact that only Section 702 was up for a vote, not the entire Foreign Intelligence Surveillance Act.

With that being said, I have personally directed the fix to the unmasking process since taking office and today's vote is about foreign surveillance of foreign bad guys on foreign land. We need it! Get smart!

Nearly two hours later he attempted to course-correct. At 9:14 a.m., @realDonaldTrump seemed to try to clarify his stance via Twitter, saying: "With that being said, I have personally directed the fix to the unmasking process since taking office and today's vote is about foreign surveillance of foreign bad guys on foreign land. We need it! Get smart!"

The subsequent tweet seemed to show a better grasp of what Section 702 permits. According to the explanation on the House Intelligence Committee's official website, Section 702 "authorizes the Intelligence Community to target the communications of non-US persons located outside the United States for foreign intelligence purposes."

What exactly does that mean, and does 702 allow the surveillance of American citizens, even incidentally? Here are some of the answers, including analysis from Joshua Geltzer, the former senior director for counter-terrorism at the National Security Council and current executive director of the Institute for Constitutional Advocacy and Protection at Georgetown University Law Center.

What is FISA Section 702?

It's just one part of the Foreign Intelligence Surveillance Act. The section specifically "allows the government to obtain the communications of foreigners outside the United States, including foreign terrorist threats," as laid out in the House Intelligence Committee FAQ sheet. For example, the program allows the government to obtain the emails or phone calls, without a warrant, of a non-American ISIS member who is outside the United States and might be plotting an attack. The committee FAQ points out that Section 702 does not allow the government to target the communications of any American, even if that American is a terrorist.

In a fact sheet, the Office of the Director of National Intelligence says there are strict guidelines on the use of 702, which "cannot be used to intentionally target any US citizen, or any other US person, or to intentionally target any person known to be in the United States. Section 702 cannot be used to target a person outside the United States if the purpose is to acquire information" from a person inside the United States.

As House Speaker Paul Ryan said on the floor Thursday morning: "This is about foreign terrorists on foreign soil. That's what this is about, so let's clear up some of the confusion here."

Geltzer emphasizes that 702 is a generalized directive from the FISA court that allows for collection from a certain foreign intelligence asset for a certain period of time, which makes it broader than a warrant. "The people targeted by 702 collection don't have constitutional protections that require a warrant anyway," Geltzer explained. But since they are using internet or phone providers with connections to the United States, investigators need some sort of authority to surveil them, which is how 702 came into play.

When has it been used?

Section 702 is billed as one of the most important legal authorities to stop terrorist attacks. FBI Director Christopher Wray said this week that "it is unbelievably valuable to the FBI because it gives us the agility that we need to stay ahead of today's rapidly changing threats." The House Intelligence Committee credits the authority with allowing the government to track, find and eliminate a top ISIS leader, Haji Iman, a process that unfolded over a two-year span in which the communications of Iman's close associates were collected.

A plot to attack the New York subway system was foiled, in large part, because of the information collected via Section 702. In 2009, federal officials arrested Najibullah Zazi, an Afghan national living in Colorado, as he was en route to New York to carry out the subway bomb plot. Zazi was discovered, according to the criminal complaint, after he corresponded with an email address used by an al Qaeda courier in Pakistan. The emails sought advice on how to build explosives and were collected under a Section 702 warrant. Zazi eventually pleaded guilty to conspiring to detonate explosives in the United States.

"Without the initial tip-off about Zazi and his plans, which came about by monitoring an overseas foreigner under Section 702, the subway bombing plot might have succeeded," according to an ODNI document describing several examples of the value of 702 data. The ODNI also cites information gathered through 702 as helping to identify a perpetrator of a 2016 New Year's Eve attack on an Istanbul nightclub, as well as helping to expose an ISIS recruiter from Trinidad and Tobago, along with his network.

"It is extremely important," Geltzer agreed. "702 collection is very important, not just for counter-terrorism, but also for foreign intelligence reasons."

Does this target US citizens?

The short answer is no. FISA Section 702 is not bulk collection, as stressed by the House Intelligence FAQ, and it cannot be used to target Americans, nor anyone of any nationality who is within the United States.

Geltzer put it simply when asked if Section 702 targets US citizens in any way: "Not only are they not allowed to be targets, but the statute explicitly bars back-door targeting."

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However, Americans' data may get picked up as part of incidental collection. That means that if an American is communicating with a foreign target, those communications could be collected. In addition, if federal authorities are already investigating a US person, they may cross-check that person's information against the 702 database.

But Geltzer pushed back against the idea that this cross-check amounts to warrantless surveillance of Americans. He argued that since the information already has been collected under 702, there is no reason for authorities to obtain a warrant to review it. "We don't generally go to courts to ask them to give authority to search that which has already been lawfully obtained," Geltzer explained.

Why do privacy advocates want to scale it back?

What's referenced above is exactly what privacy advocates are worried about. The American Civil Liberties Union, among others, has expressed great concern about some of what it sees as loopholes in the law. In a letter to the House Judiciary Committee in October, the ACLU argued that Section 702 "permits the government to unconstitutionally collect Americans' international communications without a warrant." The ACLU seems to be referring to the fact that Americans may get incidentally picked up if they are communicating with foreign targets, or that the National Security Agency may use information in US intelligence holdings to search the databases of communications already lawfully acquired through Section 702. Geltzer and the House Intelligence Committee fact sheet both note that 702 data is lawfully collected and "not the initiation of new surveillance. ... It is simply intelligence agencies reviewing the data they have already collected."

Is 'unmasking' related to all of this?

Geltzer said, "The short answer is no," and explained that unmasking occurs when intelligence analysts are looking at intelligence reporting from any number of foreign intelligence collection sources, including Section 702, and they see a US person whose name has been redacted and the standard is met to reveal the name. Section 702 does not address the standards for unmasking, and unmasking may be authorized from any number of foreign intelligence collection methods.

If Section 702 doesn't get reauthorized, what's at stake?

Former FBI Director James Comey weighed in on the importance of Section 702 on Thursday morning, tweeting, "Thoughtful leaders on both sides of the aisle know FISA section 702 is a vital and carefully overseen tool to protect this country. This isn't about politics. Congress must reauthorize it."

Geltzer put it this way: "I think the stakes are pretty high." If the generalized 702 collection were to disappear, he said, some collection might be made up from individualized FISA application. However, "702 was passed by Congress because in the modern world," he stressed, "target-by-target collection was unwieldy and impractical given the immediate nature of threats."

The heads of the CIA, the ODNI, the FBI, the Justice Department and the NSA called the program "vital" in a rare joint statement in December . "There is no substitute for Section 702. If Congress fails to reauthorize this authority, the intelligence community will lose valuable foreign intelligence information, and the resulting intelligence gaps will make it easier for terrorists, weapons proliferators, malicious cyber actors, and other foreign adversaries to plan attacks against our citizens and allies without detection," they warned.

The Senate voted Thursday to proceed to the bill reauthorizing Section 702. It will move on final passage next week, but not without a fight.