In Clan society [of Battletech universe], every clan citizen is a member of one of the five castes (warriors, scientists, technicians, merchants and laborers).
However, there's the "sixth caste", known alternatively as Dark Caste, Bandit Caste or Unproductives.
They are just what the name suggests - people who live outside the Clan society. Naturally, they're there because they don't see the Clan law as a good thing. They have no support from Clan society, and the Clans even attack Dark Caste communities sometimes.
Because they're not officially member of the clans, they're not counted in censuses, and their population (and percentage of total Clan society) is not known. Average life expectancy is approximately 36 years, much less than in Inner Sphere or in the "light" clans.