Source: US. Bureau of the Census
Release: Housing Vacancies and Homeownership
If the vacant unit is not for-rent or for-sale-only but is held for weekends or occasional use throughout the year, the unit is included in the category 'for occasional use'. Time-shared units are classified in this category if the vacant unit is not for-rent or for-sale-only, but held for use for an individual during the time of interview.
US. Bureau of the Census, Housing Inventory Estimate: Vacant Housing Units Held Off the Market and for Occasional Use for the United States [EOCCUSEUSQ176N], retrieved from FRED, Federal Reserve Bank of St. Louis; https://fred.stlouisfed.org/series/EOCCUSEUSQ176N, December 4, 2016.