The figures given are annual averages for total employment in city metropolitan areas and come from "Table 3. Employees on nonfarm payrolls by state and metropolitan area, not seasonally adjusted," of the Local Area Unemployment Statistics collected by the Bureau of Labor Statistics (BLS). Nonfarm payroll employment is a measure of the number of U.S. workers in the economy engaged in jobs excluding farm work, unincorporated self-employment, private households’ employment, proprietorships, unpaid volunteering and intelligence agency roles.
Data was not available for all 75 cities and for many cities the data given actually comprises several contiguous municipalities which make up the metropolitan area. A list of the metropolitan areas for which the BLS collects data can be found here in this report.
In some cases the same number is given for two areas because they are considered as one metropolitan area by the BLS, as in, for example, Mesa-Phoenix, Minneapolis-St. Paul, NYC-Newark-Jersey City, Los Angeles-Long Beach-Anaheim, Dallas-Fort Worth -Arlington, San Francisco-Oakland-Hayward, Denver - Aurora and Las Vegas-Henderson.