A junior officer, junior staff, or the lower ranks, referred to a low-ranking commissioned officer, typically with the rank of no greater than lieutenant. The term was often used to refer to the early part of an officer's career when he or she had little or no command experience or department head responsibility, and were sometimes otherwise colloquially known as the lower decks.
Nonetheless, exceptions to this existed: an officer with the rank of lieutenant or lower was not considered a junior officer but rather a staff officer – if he or she served as a department head or other supervisory position in a particular command's senior staff. An example of this was Ensign Harry Kim, who, despite his low nominal rank, served as the chief operations officer, the operations division department head, and part of the senior staff aboard the USS Voyager. As department head, officers who would otherwise outrank him instead served under him. The same was true for (provisional) Lieutenant junior grade and Chief Engineering Officer B'Elanna Torres. (Star Trek: Voyager).
On some starships, it was common for junior officers to be assigned to shared quarters. (Star Trek VI: The Undiscovered Country; TNG: "Lower Decks"; VOY: "Flashback", "Good Shepherd") One could request a new room assignment if they were unhappy with whom they shared a room with. (TNG: "Lower Decks") Aboard other ships, such as the USS Cerritos, junior officers only had an assigned bunk in a corridor to sleep in with limited private space. (LD: "Second Contact")
Junior officers were considered among a vessel's junior staff. One illustration of the gap between senior and junior officers occurred when, on behalf of Voyager's junior staff, Ensign Jenkins thanked Harry Kim for diffusing the situation with the series 5 long-range tactical armor unit that threatened the crew – even though they both held the same nominal rank of ensign. (VOY: "Warhead")
According to Renth, one of a trio of Takret rescued by Enterprise NX-01 in 2152, they were former members of the Takret Militia's lower ranks, who deserted after a year, when they realized that the Militia's commanding officers were corrupt. (ENT: "The Catwalk")
During the USS Enterprise-D's second encounter with Q, which involved a game that eventually had Tasha Yar sent to the penalty box, a return to the ship's bridge, with the otherwise uninvited Captain Jean-Luc Picard. While there, Yar, who was upset that if anyone else were to replace her in the penalty box would mean certain death for her, Picard issued "a new ship's standing order on the bridge: when one is in the penalty box, tears are permitted." Tasha responded, stating, "Oh, if you weren't a captain," before their interaction was interrupted by Q, who chided Picard, "Consorting with lower rank females, Captain? Especially ones in penalty boxes? Destructive to discipline, they say. But then again, you're what? You're only Human?" (TNG: "Hide and Q")
When the Romulan defector, Admiral Alidar Jarok, first arrived aboard the USS Enterprise-D in 2366, he initially claimed to be "a low-ranking logistics officer" named Sublieutenant Setal. (TNG: "The Defector")
Shortly after the USS Voyager arrived in the Delta Quadrant in 2371, Ensign Harry Kim shared with Lieutenant Tom Paris his belief of having missed an opportunity to invite Captain Kathryn Janeway to join them in the mess hall. Paris, however, felt otherwise, explaining that "Ensigns don't invite captains to sit down." When Kim asked him "Why not?", Paris simply replied, "Because they don't." Though Kim felt his intentions would have been a courteous gesture, Paris explained, "Captains don't want courtesy, they want respect. That's why they don't get chummy with the lower ranks." Whem Kim pointed out that there were, otherwise, no other captains and admirals for the captain to get chummy with, Paris, again, indicated, "If she wants to sit with us, she'll ask us to join her. That's the way it's done." Kim, nevertheless, felt Paris was "working from an old rulebook." (VOY: "The Cloud")
When Elim Garak tried to fool Worf into believing that he was interested in joining Starfleet, he claimed that "With my extensive experience I could skip the lower ranks entirely and begin my career as a commander." (DS9: "In Purgatory's Shadow")