This group consists of year-round vacant units which have been rented or sold but the new renters or owners have not moved in as of the day of interview.
Source: US. Bureau of the Census
Release: Housing Vacancies and Homeownership
US. Bureau of the Census, Housing Inventory Estimate: Vacant Housing Units Rented or Sold, Not Yet Occupied for the United States [ERNTSLDUSQ176N], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/ERNTSLDUSQ176N/, July 6, 2015.