Final answer:
A main office or branch office must be designated by the District constituting the seat of Government of the United States in a manner directed by Congress.
Step-by-step explanation:
A main office or branch office must be designated by the District constituting the seat of Government of the United States in a manner directed by Congress. This means that the government has the authority to determine how these offices are organized and established.
In the case of the U.S. Food and Drug Administration (FDA), for example, most of the Centers are located in the Washington, D.C. area as part of the Headquarters divisions. However, there are two field offices - the Office of Regulatory Affairs (ORA) and the Office of Criminal Investigations (OCI) - which have a workforce spread across the country.