A Mission Specialist is assigned to a limited field of the mission, such as for engineering diagnostics, technical assignments, or scientific researches.
Mission Specialists are considered highly adept in their specific field of specialization. They are usually division or service specific, such as in Engineering or in Science. But there are Mission Specialists that can perform specialized tasks in several areas onboard a starship or station. One such example is Seven of Nine who can perform specific missions as part of the bridge crew, or the engineering crew, or in astrometrics.