Often clients are new to certification efforts or simply intimidated by the FAA so they can and do use the services of DERs to assist them. The DERs may handle paperwork for the clients, attend meetings with the clients or act alone on behalf of the clients and much more. The FAA has many different segments that it is comprised of and often it is dfficult to know who one has to inteface with. The FAA has Flight Standard District Offices, Manufacturing and Inspection District Offices, Aircraft Certification Offices, the Military Certification Office, the Aircraft Evaluation Group, special offices for rotorcraft, engines, transport aircraft, small aircraft, etc...I have always found the FAA staff to be helpful and nice so I have a good history of working with various entities around the USA.