Mirror universe
(coexisting on another dimensional plane)
(coexisting on another dimensional plane)
In the mirror universe, lambda was a classification of Terran slaves. It was lower than the theta classification.
In 2370, Intendant Kira Nerys had a rebellious slave reclassified as a lambda and sent to the mines, although Gul Elim Garak wanted to interrogate him first. (DS9: "Crossover")