The flag honors the sacrifices made by members of the armed forces held as prisoners of war or listed as missing in action.
Facilities that are open July 4 must display the POW-MIA flag. The POW-MIA flag must fly below the U.S. flag.
For additional information, refer to the Postal Service’s guidelines on U.S. flag display and maintenance, as well as requirements for displaying the POW-MIA flag.
Source: USPS News Link