Designated survivor

In the United States, a designated survivor (or designated successor) is an individual in the presidential line of succession, usually a member of the United States Cabinet, who is arranged to be at a physically distant, secure, and undisclosed location when the President and the country's other top leaders (e.g., Vice President and Cabinet members) are gathered at a single location, such as during State of the Union addresses and presidential inaugurations. This is intended to guarantee continuity of government in the event of a catastrophic occurrence that kills the President and many officials in the presidential line of succession, such as a mass shooting or bombing. If such an event occurred, killing both the President and Vice President, the surviving official highest in the line, possibly the designated survivor, would become the Acting President of the United States under the Presidential Succession Act.

The practice of naming a designated survivor originated during the Cold War with its risk of nuclear attack. Only Cabinet members who are eligible to succeed to the presidency (i.e., natural-born citizens over the age of 35, who have resided in the United States for at least 14 years) are chosen as designated survivors. For example, Secretary of State Madeleine Albright was not a natural-born citizen (having emigrated to the United States from Czechoslovakia) and was thus not in the line of presidential succession. The designated survivor is provided presidential-level security and transport for the duration of the event. An aide carries a nuclear football with them. However, they are not given a briefing on what to do in the event that the other successors to the presidency are killed.

...
Wikipedia