"Each year, the United States Census Bureau produces and publishes estimates of the population for the nation, states, counties, state/county equivalents, and Puerto Rico. We estimate the resident population for each year since the most recent decennial census by using measures of population change. The resident population includes all people currently residing in the United States.

With each annual release of population estimates, the Population Estimates Program revises and updates the entire time series of estimates from April 1, 2010 to July 1 of the current year, which we refer to as the vintage year. We use the term “vintage” to denote an entire time series created with a consistent population starting point and methodology. The release of a new vintage of estimates supersedes any previous series and incorporates the most up-to-date input data and methodological improvements.

The population estimates are used for federal funding allocations, as controls for major surveys including the Current Population Survey and the American Community Survey, for community development, to aid business planning, and as denominators for statistical rates. Overall, our estimates time series from 2000 to 2010 was very accurate, even accounting for ten years of population change. The average absolute difference between the final total resident population estimates and 2010 Census counts was only about 3.1 percent across all counties."

https://www2.census.gov/programs-surveys/popest/technical-documentation/methodology/2010-2019/natstcopr-methv2.pdf