A Junior Officer refers to any officer not part of the Senior Staff, and may or may not be assigned to a specific department. Junior officers not specifically assigned to operations (security or engineering), science, or medical will default to a red uniform, but these officers can fill any role as needed.
Junior Officers serving on the bridge of a starship often act as relief officers for various stations. Relief Officers are expected to take over any vacant console without being assigned, as vital ship functions cannot be left attended. As such, junior officers have enough training to operate any console proficiently, even if that station's functions are not within their normal branch duties.
Other officers may be tasked with maintenance, security, damage control, and other tasks. Junior officers typically hold the rank of Ensign. Cadets will also serve as junior officers during their cadet cruise, which provides additional manpower for the ship and experience for the cadets. Lieutenant, JG and Lieutenant may serve as Junior officers, but typically have begun a transition into more senior roles. Officers at Lieutenant Commander or higher rank will have obtained a senior staff position.