Presidential line of succession is;
1) V.President,
2) Speaker of the House of Representatives,
3) President Pro-Tempore of the U.S.Senate,
-—Cabinet Level (Oldest to Newest Dept.s)
4) Secretary of State,
5) Secretary of the Treasury,
6) Secretary of Defense,
7) Attorney General,
8) Secretary of the Interior,
9) Secretary of Agriculture,
10) Secretary of Commerce,
11) Secretary of Labor,
12) Secretary of Health and Human Services,
13) Secretary of Housing and Urban Development,
14) Secretary of Transportation,
15) Secretary of Energy,
16) Secretary of Education,
17) Secretary of Veterans Affairs,
18) Secretary of Homeland Security.
So which post be he? Agriculture?
The practice of naming a designated survivor originated during the Cold War amid fears of a nuclear attack. Only Cabinet members who are eligible to succeed to the presidency (i.e., natural-born citizens over the age of 35) are chosen as designated survivors. For example, Secretary of State Madeleine Albright was not a natural-born citizen (having emigrated to the United States at age 9 from Czechoslovakia) and was thus skipped in the official 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.
So, to answer your question, I imagine that anyone after the Sec. of Labor is open to be the 'designated survivor' There was one time when Hillary was Secretary of State where she was also out of the country in London at a conference. They had a DS for that event as well, but if something happened, Clinton would have gotten the nod over the DS because her position was higher up the line of succession.
19) Secretary of Transgender Bathroom Rules.