Duty shift

A duty shift was a moment of the workday in starship duty where crewmen were effectively working. The officer in charge of the bridge during a shift was often referred to as the shift officer when the commanding officer was not on the bridge.

Three-shift rotations were common in Starfleet, as well as in the Tal Prai'ex. It was common for shift officers, in the Tal Prai'ex, to take command of the ship in battle if they had to fight while on duty.( et al.)

Alpha shift
In the Tal Prai'ex, the alpha shift was the shift where the most senior crewmen served on. It also was the "first" shift in a workday. It was also used for what technically was daytime for the crew. 

Beta shift
In the Tal Prai'ex, the beta shift was the second shift.

Rovas was the beta shift officer on the RIS. 

Gamma shift
The notion of a gamma shift, in the Tal Prai'ex, was akin to that of a night shift for land-based forces. It was usually relegated to junior officers and crewmen for it was an undesirable duty.

Ulduar was the gamma shift officer on the RIS Bouteina.