The vertical, V2, V1 and V0 ratings indicate that the material was tested in a vertical position and self-extinguished within a specified time after the ignition source was removed.
V-2: burning stops in 30 seconds on a vertical specimen; flaming particle drops are allowed.
V-1: burning stops in 30 seconds on a vertical specimen; particle drops allowed as long as they are not ignited.
V-0: burning stops in 10 seconds on a vertical specimen; particle drops allowed as long as they are not ignited.
These ratings are part of UL 94, the plastic flammability safety standard for testing parts in devices and appliances. It is a standard of flammability of plastic materials. The standard determines the tendency of the material to extinguish or spread the flame once the specimen has been ignited. UL-94 is now harmonized with IEC 60695-11-10 and 60695-11-20 and ISO 9772 and 9773.