Which Roman rule about representation did Hugo Grotius challenge?

Hugo Grotius challenged the Roman rule that restricted direct representation to specific circumstances involving slaves and dependent sons, who were viewed merely as extensions of the head of the household rather than as independent agents.12

Under this traditional Roman doctrine, free persons were generally prohibited from acting directly for another party to create legal rights and duties. In his work On the Law of War and Peace, Grotius argued that a procurator could acquire rights directly for a principal based on a mandate, maintaining that this practice did not contradict natural law. By doing so, he helped overcome the restrictive Roman conception that had long hindered the development of a formal principle of agency.12