DAV/Resourcetypes/principal

A DAV 'principal' is a type of resource used to represent a thing or group of things, generally people or computer programs, which initiate do stuff, and which therefore need to have privileges associated with them to control what stuff they can do. In the context of CalDAV, a calendar will relate to a principal, and the permissions to read and modify that calendar will be mediated by access controls applying to these 'principal' resources.

Or, from the specification in :
 * A principal is a network resource that represents a distinct human or computational actor that initiates access to network resources.