What is the primary function of the service air-line in a combination vehicle?

AAMVA CDL Manual, Section 6.2.4, p. 6-5

Confusing the service line with the emergency line is a common mix-up since both run between every unit in the combination. Knowing the service line only carries the graduated braking signal, while the emergency line supplies air and triggers the emergency brakes, is the foundation for understanding what goes wrong if the lines are ever crossed.