During landings and airborne operations, assault and paratroops were often issued large US flags to wear on their upper arms to help avoid "friendly fire" incidents. Most were discarded by the troops within days (or hours) as they a bit of a detriment to concealment. These are most often associated with airborne and glider units, but their issue was not exclusive to those troops.
Identification flags were made in several styles and materials. This version is printed on oilcloth, and is an armband that is simply wrapped around the upper arm, and secured with safety pins (included). However, some troops also cut the flag out and sewed it on.