The system used by the C.I.A. makes a strict distinction between its case officers, the clandestine employees who recruit spies abroad, and agents, those hired by the agency who often are the main source of information. Among the changes that some former officials have recommended is to establish a third category of officer to serve abroad who would be a C.I.A. employee but would have a less high-level security clearance and might never be permitted to work from the agency's headquarters.