The Second Officer is an informal position not normally recognized by Starfleet. This position is chosen by the ship or station's Commanding Officer (usually the most senior of the Department Heads), and will fill the Executive Officer if either the CO or existing XO are incapacitated, missing, or killed. If both the CO and XO are unavailable, the Second Officer assumes command.
Traditionally, the role of Second Officer is held by the most senior officer under the Executive Officer, but excluding the Chief Medical Officer. In many cases, this role falls to the Chief Security Officer, as the security and safety of the ship is already his responsibility, but other officers can fill the role depending on seniority.
There are no specific requirements for Second Officer, as the choice is informal and entirely up to the Captain. Known examples of Second Officers are as follows:
- Commander Charles Tucker
- Lieutenant Commander Montgomery Scott
- Lieutenant Commander Jadzia Dax
- Lieutenant Commander Data
- Lieutenant Tuvok