Set up by the Cousins and administered by the head of that Hive, the Servicer program allows certain criminals, deemed non-threatening, to avoid a custodial sentence by acting as a public servant available to be engaged by any member of the public.
Servicers can be considered 'unfree'; they do not have the right to self-determination, but they can leave the Servicer program and accept a standard custodial sentence. In this sense, there is a line drawn between being a Servicer and being a slave: historically, slaves could not easily leave their situation, and typically had done nothing to bring their status upon themselves. A Servicer has an alternative to service, and has committed crimes which are seen as justifying their unfree status.
Servicers are forbidden to own property of any sort, on the logic that this minimizes any motivation they may have to commit further crimes. Those who engage a Servicer may require all manner of menial duties from them, and are expected to provide a meal in exchange for their labour - the only form of payment a Servicer can expect.
The narrator of the Terra Ignota series, Mycroft Canner, is a servicer.