A tall flag carrier is part of the marching band auxiliary components that - together with other auxiliary elements - functions to add visual elements to the band's performance.
Different types of marching band have different auxiliary elements. A ceremonial marchng band, for example, might include a troop of traditional color guards; whereas corp-style field band would probably include a dance line.
The auxiliary elements of a marching band are sometimes collectively called the color guards or the visual ensemble.
I took this photo of this tall flag carrier girl practicing her skills at Bandung City Hall park a couple of days ago. I used slower shutter speed to capture the motion of the flag.
