These are the species, who have pledged to serve the Ur-Quan Kzer-Za by helping them enslave other sentient species. Their duties include providing military support for the Kzer-Za and watching over slave shielded worlds when their masters have other matters.
Battle Thralls have rights to travel through space and to retain some autonomy on their homeworlds. It is likely that their goverment's are chosen by the Kzer-Za.
They are not allowed to make war on other Thralls or "fallow" slave species. Punishment ranges from blasting villages and cities to total annihilation, depending on the severity of the crime.