This group consists of vacant units offered for rent and those offered both for rent and sale.
Source: US. Bureau of the Census
Release: Housing Vacancies and Homeownership
US. Bureau of the Census, Housing Inventory Estimate: Vacant Housing Units for Rent for the United States [ERENTUSQ176N], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/ERENTUSQ176N/, July 31, 2015.