Just watching the BBC

[Renew Deal]

23. It has to do with whether a member of congress is an "officer"

Clause 6: Vacancy and disability

In Case of the Removal of the President from Office, or of his Death, Resignation, or Inability to discharge the Powers and Duties of the said Office, the Same shall devolve on the Vice President, and the Congress may by Law provide for the Case of Removal, Death, Resignation or Inability, both of the President and Vice President, declaring what Officer shall then act as President, and such Officer shall act accordingly, until the Disability be removed, or a President shall be elected.
(Note: This clause was partially superseded by the 25th Amendment in 1967.)

The wording of this clause caused much controversy at the time it was first used. When William Henry Harrison died in office, a debate arose over whether the vice president would become president, or if he would just inherit the powers, thus becoming an acting president. Harrison's vice president, John Tyler, believed that he had the right to become president. However, many senators argued that he only had the right to assume the powers of the presidency long enough to call for a new election. Because the wording of the clause is so vague, it was impossible for either side to prove its point. Tyler took the Oath of Office as president, setting a precedent that made it possible for later vice presidents to ascend to the presidency unchallenged following the president's death. The "Tyler Precedent" established that if the president dies, resigns or is removed from office, the vice president becomes president.

The Congress may provide for a line of succession beyond the vice president. The current Presidential Succession Act establishes the order as the speaker of the House of Representatives, the president pro tempore of the Senate and then the fifteen Cabinet secretaries in order of each department's establishment. There are concerns regarding the constitutionality of having members of Congress in the line of succession, however, as this clause specifies that only an "officer of the United States" may be designated as a presidential successor. Constitutional scholars from James Madison to the present day have argued that the term "officer" excludes members of Congress.

The 25th Amendment explicitly states that if the president dies, resigns or is removed from office, the vice president becomes president, and also establishes a procedure for filling a vacancy in the office of the vice president. The Amendment further provides that the president, or the vice president and Cabinet, can declare the president unable to discharge his duties, in which case the vice president becomes Acting president. If the declaration is done by the vice president and Cabinet, the Amendment permits the president to take control back, unless the vice president and Cabinet challenge the president and two-thirds of both Houses vote to sustain the findings of the vice president and Cabinet. If the declaration is done by the president, he may take control back without risk of being overridden by the Congress.

https://en.wikipedia.org/wiki/Article_Two_of_the_United_States_Constitution#Clause_6:_Vacancy_and_disability