What is a 'designated survivor'? Sonny Perdue's State of the Union role explained

Every member of President Trump’s Cabinet was invited to attend his first State of the Union address on Tuesday night – except one, Secretary of Agriculture Sonny Perdue.

Perdue was named "designated survivor" just hours before Trump is expected to step up to the podium to deliver his address to Congress.

Ahead of the speech, Perdue will be taken to a secure and undisclosed location outside of Washington, D.C., where he is expected to stay with Secret Service agents until the conclusion of the event. When Trump and his Cabinet members safely exit the packed House chamber, Perdue will be allowed to return home.

Here’s what you need to know about the "designated survivor," a unique role that has been around for decades.

What does it mean to be the "designated survivor?"

During major political events, such as a presidential inauguration or State of the Union address, an official within the presidential line of succession is given the role of "designated survivor" and has to stay away from the event.

This precaution is taken to assure continuity in the presidency.

If a catastrophic event were to strike the Capitol and result in the death or disablement of the president, vice president, speaker of the House or other White House officials, then the "designated survivor" could step in to be the acting commander-in-chief.

When did this tradition begin?

This tradition dates back to the 1960s amid concerns of a potential nuclear attack during the Cold War. 

But official records of "designated survivors" weren't kept until 1984, starting with Secretary of Housing and Urban Development Samuel R. Pierce Jr. during Ronald Reagan's State of the Union speech on Jan. 25, 1984.

Who has been chosen as the "designated survivor" over the years?

U.S. Secretary of Agriculture Sonny Perdue speaks at an event to celebrate the re-introduction of American beef imports to China, in Beijing, China June 30, 2017. REUTERS/Mark Schiefelbein/Pool - RC1259FB7260

U.S. Secretary of Agriculture Sonny Perdue named "designated survivor" during Trump's SOTU speech on Jan. 30, 2018.  (REUTERS/Mark Schiefelbein/Pool)

Perdue has been appointed the "designated survivor" for Trump's 2018 address. As secretary of agriculture, Perdue would ordinarily rank low in the line of succession to the presidency –  ninth on a list of 17 officials.

It's not unusual for a lesser-known Cabinet member to be selected, as the president may point out higher profile officials as he mentions specific tasks and initiatives in his speech.

Department of Veterans Affairs Secretary David Shulkin was given the role during Trump's address to a joint session of Congress in 2017.

Here's a list of Cabinet members who were named "designated survivors" and did not attend the State of the Union over the past 10 years:

Jan. 30, 2018: Secretary of Agriculture Sonny Perdue

Jan. 12, 2016: Secretary of Homeland Security Jeh Johnson

Jan. 20, 2015: Transportation Secretary Anthony Foxx

Jan. 28, 2014: Energy Secretary Ernest Moniz

Feb. 12, 2013: Energy Secretary Steven Chu

Jan. 24, 2012: Agriculture Secretary Tom Vilsack

Jan. 25, 2011: Interior Secretary Ken Salazar

Jan. 27, 2010: Sec. of State Hillary Clinton and HUD Sec. Shaun Donovan

Jan. 28, 2008: Secretary of the Interior Dirk Kempthorne

Jan. 23, 2007: Attorney General Alberto Gonzales