From the IRS Website: "Migration data for the United States are based on year-to-year address changes reported on individual income tax returns filed with the IRS."
Metro-level statistics reported in the map represent an aggregate of county level migration data for the top 100 metros. An important point to note is that this excludes flows between counties within the same metro and to/from foreign countries, but includes flows to and from non-metro areas. Therefore, this analysis can be best classified as a look at metro-focused domestic migration.
Total households is from the 2010 Census, and acts as a approximate denominator for individual income tax returns to give an idea of scale relative to the metro's size.