Categories, ranges, and/or variables within pre-tabulated estimates may be updated with a data release to reflect changes and modifications to better capture social, economic, housing, and demographic trends. For example, "Subway or elevated" was line 11 in Table B08006 in 2018, but "Subway or elevated rail" was line 10 in the same table for 2019.
With every release, there are published Table and Geography Changes to share these updates, and the Table Shells and Table List provide table layouts (Table Shells) and a list with table IDs, table titles, table universes, and 1-year/5-year availability in one spreadsheet (Table List).