A directorate is defined in RFC 2418 in the following way:
In many areas, the Area Directors have formed an advisory group or directorate. These comprise experienced members of the IETF and the technical community represented by the area. The specific name and the details of the role for each group differ from area to area, but the primary intent is that these groups assist the Area Director(s), e.g., with the review of specifications produced in the area.
Some directorates cover a whole area. Some directorates are focused on a specific technology.