The IRS Migration of Exemptions data represents the net gain or loss in the number of exemptions reported to the IRS by each state. It approximates the number of people, not just tax filers, and includes family members who are listed as exemptions on federal income tax returns. The data given here is derived by calculating the difference between the reported outflow and inflow of exemptions reported by the IRS.
According to the IRS, "Migration data for the United States are based on year-to-year address changes reported on individual income tax returns filed with the IRS. They present migration patterns by State or by county for the entire United States and are available for inflows—the number of new residents who moved to a State or county and where they migrated from, and outflows—the number of residents leaving a State or county and where they went." SOI Tax Stats - Migration Data
For more information about methodology see the IRS Migration Data Users Guide.