There is a fundamental difference between Roles and Positions,
Positions are used in a hierarchical organization tree for planning purposes. Not all positions are allways occupied. Position to persons is a one-to-one relationship.
Roles on the other hand are functional descriptions of a particular job function, such as Purchasing Agent, Director, Finance Clerk, CIO, etc... Roles MUST have a "roleOccupant" to be active. Roles can be delegated. Roles have privileges. All Roles (i.e. Director) have the same privileges, but are restricted to the Organizational Unit. This makes the management of Privileges much easier.
Persons can hold more than one role to perform their day to day activities, and will inherit the privileges assigned to the specific Role.