Every Project names a responsible lead, and principalInvestigatorId is the reference that records who that is. It links to a User record, so the PI is one of the people in your tenant, and from the User's side this appears as their projectsAsPI — the set of projects they lead. The principal investigator is the archaeologist accountable for the fieldwork, the methods, and the reporting on the project, so this link drives who is shown as the lead in project listings and who owns the work in compliance documentation. The field is required: a Project always has exactly one principal investigator.
Technical section
Data model, API, and internals — visible to developers.