Final answer:
The lowest temperature at which a flammable material's vapors will ignite is called the flash point. Different substances have different flash points, categorized from below 73 °F to above 200 °F, indicating the risk level of ignition.
Step-by-step explanation:
The lowest temperature at which flammable materials' vapors will ignite when exposed to a spark is known as the flash point. A spark will not ignite the vapors at any temperature below the flash point. According to the reference information provided: Category 4 substances have a flash point below 73 °F. Category 3 substances have a flash point below 100 °F.
Category 2 substances have a flash point above 100 °F but not exceeding 200 °F. Category 1 substances have a flash point above 200 °F. Category 0 substances will not burn. Regarding safety, products with a warning such as "Store only at temperatures below 120 °F (48.8 °C). Do not incinerate." are indicating that higher temperatures may increase the pressure inside the container and exceed the flash point, posing a risk of ignition or explosion.