A provisional (or brevet) officer is a member of a military or service organization who has received a temporary commission, field commission (or field promotion) to hold a rank as a commissioned or non-commissioned officer in that organization.
Starship captains are given much leeway in interpreting Starfleet regulations and assigning crew positions. Wesley Crusher was given a field promotion as an acting ensign by Captain Jean-Luc Picard (TNG: "Where No One Has Gone Before"). The captain later field-promoted him to full ensign, recognizing Wesley's academic accomplishments and sacrifice in a rescue mission saving William Riker, Deanna Troi, and Lwaxana Troi. (TNG: "Ménage à Troi", "The First Duty")
These types of promotions are done without the approval of the organization's central authority, but by commanding officers who operate outside communication with authority. For example, Captain Kathryn Janeway awarded provisional ranks to several capable officers of a Maquis vessel in 2371 when Voyager became stranded in the Delta Quadrant. (VOY: "Caretaker")
- The provisional crewmen aboard Voyager wore a lozenge-shaped rank pin with black or gold stripes rather than the standard pips used to show Starfleet rank.
In 2366, Vice Admiral J.P. Hanson gave Commander William T. Riker a field promotion to the grade of captain. Riker chose to resume his previous rank and position as first officer when Captain Jean-Luc Picard was recovered. (TNG: "The Best of Both Worlds, Part I" and "Part II")
- Riker wore regular 24th century captain uniform insignia, four gold pips, when he was field promoted.
On the USS Valiant in 2373, the seven regular officers were killed in a skirmish with a Cardassian battle cruiser leaving the Red Squad cadets to run the ship. Many were field promoted by Tim Watters to various ranks after he assumed command of the vessel. (DS9: "Valiant")
- The cadets wore standard officer insignia on their cadet uniforms to represent their temporary promotion.