Geographic areas into which the United States and its territories are divided for the Immigration and Naturalization Service’s field operations or one of three overseas offices located in Rome, Bangkok, and Mexico City. Each District Office, headed by a District Director, has a specified service area that may include part of a state, an entire state, or many states. District Offices are where most USCIS field staff are located. District Offices are responsible for providing certain immigration services and benefits to people resident in their service area, and for enforcing immigration laws in that jurisdiction. Certain applications are filed directly with District Offices, many kinds of interviews are conducted at these Offices, and USCIS staff is available to answer questions, provide forms, etc.
Source: U.S. Citizenship and Immigration Services