Donald Trump is set to receive his first intelligence briefing on Wednesday. Under the rules set up by the Office of the Director of National Intelligence, a major party presidential nominee can bring two advisers with him as he or she is briefed on threats facing the United States.

Trump chose retired Lt. Gen. Mike Flynn, the former Defense Intelligence Agency director, who would likely hold a national security post in a Trump administration, and New Jersey Gov. Chris Christie, a longtime friend, who was first of his former rivals to endorse him, and is seen as a possible future attorney general.

The odd man out is the person who, if Trump is elected, would be a heartbeat away from the presidency: vice presidential nominee Indiana Gov. Mike Pence.

Why no Pence?

First is the ODNI limitation that the briefing is for the candidate and just two others. Second is the fact that the briefings are conducted in the location where the presidential nominee is campaigning, typically in a local FBI field office.

Pence is campaigning for Trump in Nevada today. Trump is in New York.

Whether the two candidates on the ticket are briefed together or separately is basically up to the campaigns, according to a spokesman for the ODNI. But vice presidential candidates can receive their own briefings if, due to scheduling or logistics, they are unable to attend with the presidential nominee.

The briefings are not done by the nation's top spymaster, Director of National Intelligence James Clapper, but by a lower-level career intelligence officer.

The idea is to ensure the briefings are nonpartisan, not conducted by a political appointee, such as Clapper

The topics covered include the major threats such as Russia, China, North Korea, Iran and terrorism, plus areas that the candidate asks about specifically.

Trump's selection of Flynn and Christie are an indication of who he feels comfortable getting his national security advice from.

Because the intelligence is limited to conclusions and assessments, it it not required that either the candidate or the advisers have a security clearance, just as members of Congress don't need a security clearance to attend a classified closed-door briefing on the hill.

While there has been hand-wringing on both sides about whether the candidates can be trusted with sensitive national intelligence, sources familiar with the briefing process say the most valuable secrets are not revealed until the candidate is elected.

For his part, Trump has expressed skepticism about the value of U.S. intelligence.

In an interview that aired on Fox News the morning before his first brief, he said he might not even rely on the judgment of intelligence professionals.

"I won't use some of the people who are sort of some of your standards. You know just use them, use them, use them. Very easy to use them, but I won't use them, because they've made such bad decisions."

As is often the case with Trump, it's not entirely clear who he was referring to as "your standards."