Emsi’s veteran data comes from the five year 2015 ACS data and includes veteran counts by county for the United States.
As defined by the ACS, veterans are men and women who have served (even for a short time), but are not currently serving, on active duty in the U.S. Army, Navy, Air Force, Marine Corps, or the Coast Guard, or who served in the U.S. Merchant Marine during World War II. People who served in the National Guard or Reserves are classified as veterans only if they were ever called or ordered to active duty, not counting the 4-6 months for initial training or yearly summer camps.
ACS five year data has a two-year lag between when the data is collected and when it is released (i.e. late 2017 data run would include 2011-2015 ACS data). This data is typically updated during a late year data run.