The ACS uses the concept of "current residence" to determine who should be considered residents of sample housing units. The basic idea behind this concept is that everyone who is currently living or staying at an address for more than two months is considered a current resident of that address. This means that their expected length of stay is more than two months, not that they have been staying in the housing unit for more than two months at the time when the survey is conducted.

 Persons away from their residence for two months or less, whether in the United States or overseas, on a vacation or on a business trip, are considered to still be a "resident" at the address, and the unit is classified as occupied and eligible for inclusion in the survey.

 Persons away from their residence for more than two months are considered not to be a resident. For the ACS, if no one is determined to be a current resident in the sampled housing unit, it is classified as "vacant."

 You can learn more about the history of the ACS in Chapter 6 of the Design and Methodology Report.