They describe the fire behaviour of materials according to the UL 94 testing. It is differed to the following classes:

HB (Horizontal Burn)
During this test method the sample is lighted horizontal at one ending. The burning rate must not be higher than 76mm/mm (at a sample thickness of more than 3 mm it is 38 mm/min).

V-2 up to V-0 (Vertical Burn)
The sample is positioned vertical and is lighted at the bottom. This testing is stricter than 94 HB. It is divided into three classes: 94V-0, 94V-1 and 94V-2

V-2
Self-extinguishing not later than 30 seconds, burning drops accepted

V-1
Self-extinguishing not later than 30 seconds, burning drops not accepted, after-glowing of max. 60 seconds

V-0
Self-extinguishing not later than 10 seconds, burning drops not accepted, after-glowing of max. 30 seconds

5V
Increased fire test. A vertical positioned sample is treated 5 times each 5 minutes with a 127 mm high flame. After the fifth time after-glowing which lasts more than 60 seconds or burning drops are not accepted.

5VB
Like 5V, with a horizontal sample. Burning holes after extinguishing are accepted.

5VA
Like 5VB, but no burning holes accepted.