Where Used (for forms only) The where used column identifies the primary users of a form.   AC Air Carrier AE Air Exchange Office AM Airport Mail Facility AO Associate Office AR Area Office BC Bulk Mail Center CA CAG A–L Post Offices CD Customer Service and Sales District CE Facility Service Center CR Post Offices with City and Rural Delivery DN Distribution Networks Office HQ Headquarters IC Inspector–in–Charge MB Mailbag Depository and Repair Center MD Material Distribution Center MSS Mailing and Shipping Services NCS National Customer Support Center OIG Office of Inspector General PC Processing and Distribution Center PE Postal Employee Development Center PH Purchasing Service Center PO All Post Offices PS Postal Service–Wide PU Public PV Post Offices with Vehicles SC Information Service Center SD Postal Source Data System (PSDS) Office STF Stamp Fulfillment Services TC National Test Administration Center VM Vehicle Maintenance Facilities XO Surface Exchange Offices