Which word must appear below an officer's signature when they are temporarily succeeding to command in official correspondence?

When an officer is temporarily succeeding to command in official correspondence, the term that must be used below their signature is "Acting." This terminology is a standard protocol within military correspondence as it communicates clearly that the officer is not the permanent holder of that command position, but is fulfilling those responsibilities temporarily.

Using "Acting" designates the nature of their authority in that role and helps to avoid any confusion regarding their command status. In official communications, clarity is crucial, especially in hierarchical structures like the Marine Corps. This designation also serves to maintain organizational continuity and proper chain of command, as it implicitly informs the recipients of the correspondence about the officer's temporary authority and the likelihood of a change in leadership in the near future.