In most militaries, a Lieutenant is a platoon or lance commander. In naval or administrative settings, they command departments or sections. The Naval equivalent to this rank in most militaries is Ensign.
In the SLDF many Lieutenants were "mustangs", non-commissioned officers who were so capable that they went to Officer's Candidate School. Unlike many other militaries, the SLDF did not immediately commission graduates of military academies as Lieutenants. These recent graduates were instead made Sergeants until they distinguished themselves on the battlefield.
SLDF Lieutenants wore a simple olive green bar as their insignia.
In the second Star League, Lieutenants wore the same insignia as their first Star League counterparts. The second Star League also used an additional Lieutenant rank, Lieutenant, Senior Grade. This rank was indicated by adding a silver pip to the center of the standard Lieutenant insignia.
In the LCAF, a Leutnant is the rank given to recent academy graduates while they demonstrate their technical and leadership skills. Uniquely, the Leutnant has not yet won a commission, and is technically not an officer, similar to the AFFS rank of Subaltern. A Leutnant is frequently referred to by their job title, for example, MechWarrior, Gunner, Pilot, etc. 
Once a Leutnant demonstrates their leadership abilities, they are promoted to the rank of First Leutnant, and given permanent command of a lance or platoon.
In the DCMS, a lance or platoon leader holds the rank of Chu-i. These officers wear a royal blue katakana number 3 as their rank insignia. In the Draconis Combine Admiralty these officers are called Sho-i (Ensign). A sho-i who commands a lance of two AeroSpace fighters is often called a lancer. In the DCMS Support and Engineering Corps (Technical Service) officers at this rank are known as Master Chief Petty Officers.
Formerly known as Subcommander, post-Xin Sheng the CCAF retitled this rank as Sao-wei or Lieutenant. After graduating from the academy all MechWarriors and fighter pilots are given the rank of lieutenant. Among conventional forces a lieutenant commands an infantry platoon or armor lance, or serves in a managerial position in non-combat units.
Within the elite Warrior Houses, the equivalent rank is Ban-zhang, denoting an infantry squad leader or MechWarrior.
Traditionally, the FWLM divides Lieutenants into two ranks: Lieutenant, Junior Grade (Lieutenant JG) and Lieutenant, Senior Grade (Lieutenant SG). In any case Lieutenants serve as lance/platoon leaders or staff officers.
Typically Lieutenant SG's were lance commanders, while Lieutenant JG's were recent academy graduates (similar to the AFFS Subaltern).
Lieutenant JG's wear a single narrow gold bar as their rank insignia, while Lieutenant SG's wear a wide gold bar and a narrow gold bar as their rank insignia.
ComStar Adepts use a ComStar logo with a blue halo to show their rank. Word of Blake Adepts use a silver medallion. Insignia shows theta symbol, indicating JumpShip/WarShip crew. Other service branches replace theta with appropriate Greek letter.
In the KungsArmé officers with this rank are called Lojtnant, Swedish for Lieutenant. These soldiers wear a green patch with a vertical gold bar crossed at the base by a small gold bar as their rank insignia.
In the Magistracy Armed Forces, all officer ranks are purchased, with the rank of Ensign as the lowest for sale, anywhere from between 10,000 to 15,000 C-Bills. Found in both the Army and Navy, an Ensign is not assigned the command of a specific number of personnel, but varies depending on the mission, circumstances and her combat experience. Some ensigns can command as few as ten or as many as a hundred.
Ensigns wear a blue-green diamond edged in gold as their rank symbol.
In the Taurian Defense Force, a Cornet is the lowest-ranking junior officer. Among the conventional forces Cornets command companies of infantry or armored vehicles. Prior to 3065, all MechWarriors officially attained the rank of Cornet, though afterwards it became reserved for 'Mech lance commanders. The TDF naval equivalent is Ensign, and either serve as assistant department heads aboard ships or command a Wing of two air lances. These officers wear a single gold ring as their rank symbol.
The Legiones Marianes, with their focus on re-creating the Roman Empire, do not use this rank. The Latin name for the lowest level of HAF officers is Legionnaire. Senior Legionnaires (Lance commanders) are known as Centurions. A legionnaire wears a silver triangle as his rank insignia, while a centurion wears a silver triangle topped by a black triangle with a small silver triangle in the center.