The tradition of keeping at least one cabinet member away from the State of the Union Address has been explored by the world of entertainment. The idea is that in the event of a horrific disaster or attack on the prime-time event, a clear line of presidential succession is maintained.
The practice of designating a survivor isn't laid out in the Constitution, but the presidential line of succession is covered in Article II, Section 1 of the Constitution and the Presidential Succession Act. But neither requires the sequestration of officials. "A Senate-confirmed Cabinet official should be designated to sit out the State of the Union". Trump's choice this year was limited by the number of "acting" secretaries in the Cabinet.
At his confirmation hearing in 2017, Perry said he regretted recommending the elimination of the Energy Department.
The practice originated at some point during the Cold War, but it took on renewed resonance after the September 11 attacks. For instance, Lu recalled that he requested Eric Holder, then the attorney general, to serve as designated survivor for Obama's February 2009 address to a joint session of Congress, which focused on Obama's push to pass the economic stimulus bill.
The 1947 law also notes that the line of succession would only apply to those who would be constitutionally eligible to serve as president.
But this year, the pool of eligible survivors is a bit shorter than usual due to multiple vacancies in the president's cabinet.
One cabinet member, Transportation Secretary Elaine Chao, is ineligible under the constitution because she was not a US citizen at birth.
Patrick Shanahan now serves in the role of acting Defense Secretary.