Final answer:
Among the sources used to define an EMT's scope of practice, it is the EMS system's medical director who ultimately delineates their scope of practice within a specific organization or region.
Step-by-step explanation:
The scope of practice for an individual Emergency Medical Technician (EMT) is influenced by several factors, one of which is the direction and oversight from the EMS system's medical director. While the EMT textbook and other training materials certainly contribute to an EMT's knowledge base, and the United States Department of Transportation's Emergency Response Guidebook may serve as a helpful resource in certain situations, they do not define the scope of practice. Similarly, the National Registry of Emergency Medical Technicians certification exam assesses an EMT’s competency, but it is the EMS system's medical director who ultimately delineates the scope of practice within a specific region or organization.