In Vulcan and Romulan ranks, a sublieutenant was a junior officer rank, equivalent to the Starfleet rank of lieutenant.
Before revealing his true identity, Alidar Jarok referred to himself as Sublieutenant Setal, a low-level logistics officer. (TNG: "The Defector")
Administrator V'Las had a security officer who was a sublieutenant. (ENT: "Kir'Shara")
The name of this rank was formatted "Sublieutenant" in the script of "The Defector"  but "Sub-Lieutenant" in the script of "Kir'Shara".