The homeownership rate is computed by dividing the estimated total population in owner-occupied units by the estimated total population (ACS 5-year variables B25008_002E and B25008_001E from table B25008, respectively).
A housing unit is owner-occupied if the owner or co-owner lives in the unit, even if it is mortgaged or not fully paid for. A housing unit is classified as occupied if (i) it is the current place of residence of the person or group of persons living in it at the time of interview or (ii) the occupants are only temporarily absent from the residence for two months or less (e.g., on vacation or a business trip). If all those staying in the unit at the time of the interview plan to stay there for two months or less, the unit is considered to be temporarily occupied and classified as "vacant."
Multiyear estimates from the American Community Survey (ACS) are "period" estimates derived from a data sample collected over a period of time, as opposed to "point-in-time" estimates such as those from past decennial censuses. ACS 5-year estimate includes data collected over a 60-month period. The date of the data is the end of the 5-year period. For example, a value dated 2014 represents data from 2010 to 2014. However, they do not describe any specific day, month, or year within that time period.
Multiyear estimates require some considerations that single-year estimates do not. For example, multiyear estimates released in consecutive years consist mostly of overlapping years and shared data. The 2010-2014 ACS 5-year estimates share sample data from 2011 through 2014 with the 2011-2015 ACS 5-year estimates. Because of this overlap, users should use extreme caution in making comparisons with consecutive years of multiyear estimates.
Please see "Section 3: Understanding and Using ACS Single-Year and Multiyear Estimates" on publication page 13 (file page 19) of the 2018 ACS General Handbook for a more thorough clarification. https://www.census.gov/content/dam/Census/library/publications/2018/acs/acs_general_handbook_2018.pdf