Subofficer is an enlisted rank in the Cardassian military. The rank was sometimes shortened to Sub, as Berat addressed Halek as "Sub Halek" on several occasions. (DS9novel: Betrayal)
Halek was a Subofficer aboard theSwift Striker. Whether the rank of Subofficer was always used by the Cardassians (it could be another word for garresh), or just when the Revanche party came to power, is unknown. It is also unknown what the Starfleet equivalent is.