An advocaat is a lawyer. I am often surprised at the number of court cases my ancestors were involved in. Even some of my serf ancestors hired lawyers to defend their rights to cutting trees or taking over the farm against the landlord.
Most lawyers would have had a university education. Since 1575, Leiden had a university where people could get a law degree. Other universities followed later. You will recognize these academically trained lawyers by the letters “J.U.D.” behind their names (Juris Utriusque Doctor, Latin for doctor in both civil and canon law).