A housing unit which is occupied temporarily by persons who usually live elsewhere is interviewed as a vacant unit provided that a usual place of residence is held for the household which is not offered for rent or for sale. For example, a beach cottage occupied at the time of the interview by a family which has a usual place of residence in the city is included in the count of vacant units. Their house in the city would be reported "occupied" and would be included in the count of occupied units since the occupants are only temporarily absent. Units occupied by persons with usual residence elsewhere (URE) are further classified as seasonal vacant or year round vacant units.
Source: US. Bureau of the Census
Release: Housing Vacancies and Homeownership
US. Bureau of the Census, Housing Inventory Estimate: Vacant Housing Units Held Off the Market and Usual Residence Elsewhere for the United States [EUREUSQ176N], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/EUREUSQ176N/, November 25, 2015.