The CTU Chief of Staff was the second-in-command of a CTU Domestic Unit. Often called Deputy Director, or simply "Number Two". If the Director of a domestic unit was not available or unfit for command, the Chief of Staff assumed their responsibilities.
When Tony Almeida was promoted to Director during Day 2, Michelle got promoted to Chief of Staff. She held the position through most of Day 3. However, when Tony was shot in the neck, Michelle became acting Director. After the events of Day 3, Michelle left CTU Los Angeles for CTU Seattle and eventually received a position at Division.