You need to refuel your truck while it is loaded with placarded hazardous materials. What are you required to do during fueling?

49 CFR 397.15 (Fueling); AAMVA CDL Manual Sec 9.6.7 (nhcdm.pdf p.9-14)

Fueling combines two risks at once: an ignition source in the engine and flammable vapors from the fuel and possibly the cargo. Shutting the engine off removes one risk, and keeping a person actively at the nozzle means fuel flow can be stopped instantly if something goes wrong, instead of discovering an overflow or leak after the fact.