one of the main functions of the cartórios, is to provide authentication of signatures, and of official documents, or deeds of any kind - this service is known as the serviços de notas, and is done by the notaries that work in the offices. 

one of the very first things anyone who is planning on doing anything in rio de janeiro that would require documentation and notarizations should do, is going ahead of everything to notarize and authenticate his or her own signature at a convenient notary office, a procedure called certification of signature.

most legal documents require an authenticated signature, and a cartório near the place of residence would be ideal for this purpose - another particularly prominent service, provided by the cartórios is just about anything to do with the registering, purchasing, or selling of property and real estate.