Trump got a 'target letter.' What is one and what does it mean?

With sources saying special counsel Jack Smith has sent former President Donald Trump a "target letter," what does that mean, as a general matter ?

An official notice, a target letter, according to Justice Department guidelines and experts who spoke with ABC News, is typically the last step in an investigation before an individual is charged.

"A target letter, as a technical matter, is notification that the government has substantial evidence linking you to a crime and that you are a quote, putative defendant," Widge Devaney, a former federal prosecutor said. "As a practical matter, it means we are almost certainly going to charge you, and if you have anything to say to us say it now."

An example target letter posted on the Justice Department website outlines that if an individual is informed that if he or she wants to testify before the grand jury, they have a final opportunity.

"You are advised that you are a target of the Grand Jury's investigation," the example target letter says. "You may refuse to answer any question if a truthful answer to the question would tend to incriminate you. Anything that you do or say may be used against you in a subsequent legal proceeding."

Former federal prosecutor Joseph Moreno told ABC News it is highly unlikely a target of an investigation would choose to testify.

"In theory, a target can come in and convince a grand jury why charges would not be appropriate. In reality, this is rarely successful and if anything the target has now provided incriminating responses which are admissible at trial," Moreno said. "For these reasons defense counsel will almost always advise their clients not to speak with a grand jury or anyone for that matter, as it is significant downside and very little upside."

When Trump was charged by Smith in the national security documents case, he received a target letter shortly before he was indicted.

However, there is no rule on the time frame in which a target letter can be sent, according to Devaney. Trump has pleaded not guilty in the documents case.

"I think in this particular instance, I would just look back to that one precedent we have, this was sort of a final notice that that's how Smith used the target letter," he explained, adding it is "rare" for someone to get a target letter and not get charged.