UL 94 classifies plastics by how specimens behave under two standard flames. The ratings are comparative, thickness-specific and frequently misquoted - a material is not "UL 94 material"; it has a rating at a stated thickness, from a stated test.
The vertical ratings (50 W flame)
Five vertical specimens are flamed twice for 10 seconds each. V-0: no specimen burns longer than 10 s after either application, total flaming for the set stays within 50 s, no specimen burns up to the clamp, no drips ignite the cotton below, and afterglow stays within 30 s. V-1 relaxes the times (30 s per application, 250 s total, afterglow 60 s) but still forbids cotton ignition by drips. V-2 keeps V-1 timing but allows flaming drips to ignite the cotton. The ranking V-0 > V-1 > V-2 is about self-extinguishing discipline, not about how fast the bar burns.
HB and the 500 W ratings
HB comes from the horizontal test: the burn rate along a horizontal bar must stay below limits that depend on thickness (for example 40 mm/min for specimens 3 to 13 mm thick). The 5V ratings use the 500 W flame on bars and plaques: 5VB specimens may develop a hole; 5VA specimens may not. Thin-film and foam ratings (VTM, HF) have their own specimen arrangements in the same standard family.
Running the test
All of these are run in one chamber with 50 W and 500 W burners - the Motis UL94-X chamber covers both flame powers and the IEC 60695-11-10/-20 counterparts. When you compare supplier datasheets, compare rating and thickness; both belong to the result.