Able Crewperson (also "able crewman") is the term to describe an astronaut who does not happen to have a specific role or position on the mission. A notable able crewperson was Nevin Hotson, who is credited with having coined the term. In the 2007-08 astronaut application, the term "General Labour" was used instead, but it denotes the same role.
On all documented missions in Sim history, there have been able crewmen. Typically, there are up to four on a mission, though often fewer as it is likely that some astronauts will have specialized roles.
Able Crewmen are junior officers, and are typically all of the same rank (Lieutenant). This is due to the fact that more senior ranks are generally held by more senior astronauts, who have had more experience and training, and therefore can hold more specialized positions.
Able Crewmen typically have no specialized station, and switch off periodically. All stations except Pilot are usually crewed at one time or another by Able Crewmen. Able Crewmen are sometimes classified as Payload Specialists in error. However, Payload Specialists always have a specific function associated with one or more experiments.