A protocol is a group of notarial records for a year, and “Protocolos Notariales” is the name most used in Spain. They include all documents certified by notaries, for example: rent agreements, wills, dowries, mortgages, donations, guardianships, sales of land or goods and settlements.
We are discussing the Notarial Records in Spain but the records in Portugal and Latin America are similar. When the notary retires he transfers the documents to the notary replacing him and they are only available to the family of the person that requested the document. All documents older than 100 years are kept at the at the Historical Archive of the corresponding province (Archivos Históricos Provinciales), except for Madrid, where they are kept in a specific Archive (Archivo de Protocolos). The documents in the Archives are public records and are available to any researcher.
Notarial Records are hard to use because of the amount and diversity of the documents. But these documents provide useful information for the genealogical researcher, for example wills include the names of the live descendant, and dowry documents always mention the bride parents (and sometimes the grandparents).
The writing is usually very hard to understand, as notaries utilized the “notarial” handwriting. In many locations these documents go back until the XVI century, but the older the document, the harder to read.