Final answer:
As a Soldier progresses in rank and becomes an NCO, they add gold or yellow stripes to their Army Blue Service Uniform trousers, representing their new leadership status and their advancement within the military ranks.
Step-by-step explanation:
When a Soldier progresses in rank and enters the Noncommissioned Officer (NCO) Corps in the U.S. Army, something significant is added to their Army Blue Service Uniform to mark this rite of passage. Upon being promoted to the NCO ranks, a Soldier is distinguished by the addition of the NCO Professional Development Ribbon and, more visibly, the iconic gold or yellow stripes on their blue trousers, commonly known as 'NCO stripes'. These stripes are a clear indication of their leadership role and their progression within the military hierarchy.