McMaster needs Senate confirmation because he's a 3-star general

— -- The position of national security adviser does not require Senate confirmation, but when three-star generals like Trump's pick for the position, H.R. McMaster, change jobs within the service they do need the approval.

The rule has nothing to do with the White House, but rather the military: all three and four-star generals must receive Senate confirmation whenever they seek to change jobs.

What does this mean for McMaster?

If McMaster wanted to keep his lieutenant general status, President Donald Trump would need to reappoint him and then wait for him to get a confirmation vote on the Senate floor, according to a Senate Armed Services Committee aide.

This happened to Gen. Colin Powell when he served as President Ronald Reagan's national security adviser. The Armed Services committee held a hearing on him, then referred his nomination to the Senate, and then the full Senate voted on him.

But that doesn't necessarily mean McMaster has to go through the confirmation gauntlet.

There are two options that would allow him to serve as national security adviser without needing to be confirmed, according to the committee aide: He could revert to two-star major general status to lead Trump's National Security Council, or he could retire from active military duty.

Those were the options Powell faced. In the end, as he wrote in his memoir, "My American Journey," he chose to keep his higher ranking and go through confirmation.

"The post of national security adviser did not require Senate confirmation. But as a three-star general, I would have to be confirmed for any job in order to hold on to my rank. If I dropped back to two stars, I could be appointed without Senate confirmation. But I was not eager to be demoted in the Army so that I could be promoted in a civilian post," he wrote.

It's not clear how many committee hoops McMaster would have to jump through if he decides to stay a lieutenant general. Committee aides would not yet comment on whether they would require a hearing, if McMaster chooses to remain a three-star general, given that the position does not otherwise require one.

At the end of today's White House briefing, press secretary Sean Spicer said McMaster would not require Senate confirmation, but did not explain further. He was not asked about the rules governing three and four-star generals.

The White House has not yet returned ABC News' request for more information.