Final answer:
The red section of the NFPA 704 marking system indicates flammability, using a scale from 0 (no hazard) to 4 (extremely hazardous).
Step-by-step explanation:
The red section of the NFPA 704 marking system indicates the level of flammability of a substance, which is how easily it can catch fire. This is represented in the upper diamond of the NFPA hazard diamond and uses a rating scale from 0 to 4, with 0 indicating no fire hazard and 4 signifying extreme flammability. The NFPA 704 system is designed to provide quick and easy safety information on chemicals, detailing flammability, reactivity, health, and more.