The CTU Chief of Staff is the second-in-command of a CTU Domestic Unit. Often called Deputy Director, or simply "Number Two". If the Special Agent in Charge of a domestic unit is not available or unfit for command, the Chief of Staff assumes his responsibilities.
When Tony Almeida was promoted to Special Agent in Charge 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 was become acting Special Agent in Charge. After the events of Day 3, Michelle left CTU Los Angeles for CTU Seattle and eventually received a position at Division.