Final answer:
An NCOER is mandatory when an NCO is in a leadership position during a TDY, SD, or TCS of 90 days or more, as it's crucial for recording performance and career development.
Step-by-step explanation:
An NCOER (Noncommissioned Officer Evaluation Report) is mandatory for TDY (Temporary Duty Yonder), SD (Special Duty), or TCS (Temporary Change of Station) when the NCO (Noncommissioned Officer) is in a leadership position and the assignment is for a period of 90 days or more. This is done to provide a record of performance and potential, important for career development and future assignments of the NCO. The evaluation is used to ensure that leaders are effectively assessed and that they receive the necessary feedback to foster professional growth during such temporary assignments or special duties.