A biohazard level in Umbrellas contamination unit.
It is never directly referred to in the series. However, in Resident Evil 2, a similar counting method was referred to as a "Class 1 Contamination". Due to the fact that both measure Contaminations, and that the Umbrella Corporation would be unlikely to have two counting methods, it is probably an alternative name.
What to do in a Class 1 Biohazard was mentioned in a file in Resident Evil 2. It consulted destroying the emergency train on level B8F if it starts moving with Bio-hazardous cargo. This Caused the train to auto-destruct upon the infected William Birkins entrance onto the train.