Flaggers from multiple organizations display their stop flags to an oncoming buggy during freeroll practice.
Each organization's chute flagger is required to have a stop flag so they can signal to their drivers that there is a hazard ahead. The stop flag must be yellow with a large, black X across its entire face, and is the only flag that may be yellow. A driver must come to a controlled stop upon being signaled with a stop flag. Usually there is also a stop flag at the transition flag as well as at the chute flag. Often during rolls, multiple flaggers will put out their stop flags when they need to signal a driver to make it easier for the driver to see the flag.
The requirement to have a stop flag is covered in Rule 8.1.3 Signal Flaggers.