A Mission Advisor does as the name suggests; advises the Commanding Officer and Executive Officer on mission-specific areas of importance. Many times, the Mission Advisor knows just as much about the mission as the CO and XO do, if not even more. He or she also performs mission-specific tasks, and can take on any roles that a mission requires him or her to do. Concurrently holds another position, except in rare circumstances.
This position it typically only utilised on a mission-by-mission basis, and is not a constant aboard a starship, except in unusual circumstances where an advisor is required for more than one mission. When the advisor is not a member of the ship's crew, they will often be a senior officer - or sometimes flag officer - with the specialised knowledge on the mission at hand who is temporarily assigned to the ship undertaking the mission.