In Starfleet, it was above Lieutenant Junior Grade and below Lieutenant Commander. It normally took a minimum of five years to earn the rank. It was the last rank that merely a clean record and time in grade an officer could earn.
Most Lieutenants served as Department Heads or assistant ones.
On small vessels, they were occasionally Executive Officers.
Highly experienced medical personnel and scientists were often given direct commissions at this rank.
They were eligible for promotion to Lieutenant Commander after a minimum of three years time in rank, as long as they had nine total years in Starfleet.
United Federation Marine CorpsEdit
The Marine equivalent rank was called Captain, and they normally commanded a company or served in staff positions.
Klingon Defense ForceEdit
In the Klingon Defense Force, it was called Sogh.
B'rel class Birds of Preys were often commanded by exceptional Sogh's.