From lowest (Least flame-retardant) to highest (Most flame-retardant):

HB: slow burning on a horizontal specimen; burning rate < 76 mm/min for thickness < 3 mm or burning stops before 100 mm

V-2: burning stops within 30 seconds on a vertical specimen; drips of flaming particles are allowed.

V-1: burning stops within 30 seconds on a vertical specimen; drips of particles allowed as long as they are not inflamed.

V-0: burning stops within 10 seconds on a vertical specimen; drips of particles allowed as long as they are not inflamed.

5VB: burning stops within 60 seconds on a vertical specimen; no drips allowed; plaque specimens may develop a hole.

5VA: burning stops within 60 seconds on a vertical specimen; no drips allowed; plaque specimens may not develop a hole.

Tests are generally conducted on a 5" x 1/2" (127 mm x 12.7 mm) of the minimum approved thickness. For 5VA and 5VB ratings, tests are performed on both bar and plaque specimens, and the flame ignition source is approximately five times as severe as that used for testing the other materials.

There are other classifications that apply to low density foam materials (HF-1, HF-2, HBF) and thin films (VTM-0, VTM-1, VTM-2).

HF-2: burning stops within 3 seconds; afterglow less than 30s; burning drips allowed.

HF-1: burning stops within 2 seconds; afterglow less than 30s; no burning drips allowed.