Final answer:
A yellow flag used by a utility operator marks the location of underground Gas & Fire Pipelines. Color-coded markings are critical for identifying and avoiding damage to various utilities.
Step-by-step explanation:
When a utility operator locates underground utilities and flags the area with a yellow flag, this color represents gas. Color-coding is essential for identifying various types of underground utilities to avoid damages during excavation work. Yellow flags or markings indicate the presence of underground Gas & Fire Pipelines, which is crucial to note for safety and preventing hazardous accidents.