What constitutes an 'official act' by a president?

In a historic ruling on Monday, the U.S. Supreme Court said former President Donald Trump is entitled to immunity from criminal prosecution for "official" acts taken as president, but not for any "unofficial" acts.

The 6-3 decision could have major implications for the various criminal cases pending against Trump -- especially special counsel Jack Smith’s federal case, for which Trump faces four felony counts for alleged attempts to overturn the 2020 election.

What constitutes an "official" versus an "unofficial" act by the president is not precisely defined in the opinion, and Chief Justice John Roberts acknowledged it could raise "difficult questions."

"Certain allegations -- such as those involving Trump's discussions with the Acting Attorney General -- are readily categorized in light of the nature of the President's official relationship to the office held by that individual," Roberts wrote in the opinion. "Other allegations -- such as those involving Trump's interactions with the Vice President, state officials, and certain private parties, and his comments to the general public -- present more difficult questions."

In addition to the core presidential duties laid out in the Constitution, conduct within the "outer perimeter" of official functions would be deemed immune as long as it is "not manifestly or palpably beyond his authority."

It would be up to the lower courts to determine whether the conduct in question is considered official or unofficial.

"[Official acts are] something that you would expect the president to do -- kind of a core presidential duty, like acting as Commander-in-Chief of the military," said Chris Timmons, a former prosecutor and ABC News legal contributor. "If the president of the United States sent troops to Lebanon, for example, he couldn't be prosecuted for murder."

Though the ruling has been largely deemed a win for Trump, it’s far from a get-out-of-jail-free card, legal experts told ABC News -- particularly when it comes to prosecution for actions he took not as the president but as a candidate.

When it comes to allegations that Trump enlisted "fake electors" to overturn the 2020 election in his favor, for example, it would likely be difficult to argue that was done in his official capacity as president.

"The interaction with fake electors is not something a president does as part of his official duties -- it is something a candidate does as part of their campaign," Michael Gerhardt, a constitutional law expert at the University of North Carolina, told ABC News. "That allows the court to say, I think rightly in this case, that Trump's interaction with the fake electors is really on the unofficial side, rather than official."

Even so, punting the decision to the lower courts is almost certain to throw obstacles in the way of the pending litigation against Trump, slowing them down even further ahead of the election.

"The court basically said that as long as Trump or any president claims that what he was doing was acting officially, then his actions are presumptively constitutional, and it's up to the prosecutor to find evidence to overcome that presumption, and that is not going to be easy," Gerhardt said.

Some legal experts expect Smith may reevaluate the federal case against Trump, possibly jettisoning some elements that could prove shakier due to the Supreme Court ruling.

"One option is to try to streamline the case considerably to only those acts that either Trump conceded were unofficial, or those acts plus some that Jack Smith thinks he has the best chance of persuading the courts are unofficial, and then proceed on that basis in the interest of efficiency," Jessica Roth, a former federal prosecutor and Cardozo Law professor, told ABC News. "Or does he want to be more aggressive and try to keep more of the allegations in the case, which might be more risky for him in terms of ultimately prevailing?"