Slavery was a common practise in the Old Kingdoms, but was abolished in Anno 7 NP. Slavery was reinstated under the Magi in Anno 720 NP.
Slaves are treated as property, and have no more rights than a Servient.
Many Slum-Dwellers sell themselves or relatives into slavery. This ensures a potentially better life for the slave, and brings some income into the family.
The vast majority of slaves are human.