Found while preparing a display on census records… British Aliens in the United States during the War of 1812 by Genealogical Publishing Co. Inc. 1979
Once war was declared, British subjects were now enemy aliens. They were given notice to report to the marshal of the state or territory.
They had to report the people composing their families, places of residence, occupations, whether and when they’d applied to the court to prepare for naturalization. These reports were to be submitted by the marshals to the Department of State. Usually the report gave the aliens names aged 14 or above, # of people in the family, place of residence and status.
There may be extra information such as the names of wives and children, whether born here or overseas, names and ages of other occupants in the house, relatives, and children of the wife by a previous marriage, and apprentices and servants.
There may even be a physical description. Depending on the state there can be even more information.