A biohazard contamination level indicator used by Umbrella.
It is never directly referred to in the series. However, in Resident Evil 2, what was likely a similar designation for a Level 1 Biohazard was referred to as a "Class 1 emergency". Due to the fact that both warnings are used to measure contamination, and that the Umbrella Corporation does not specify a distinction between the two, it is probably an alternative name.
The procedure to follow in case of a Class 1 Biohazard was mentioned in the Laboratory Security Manual in Resident Evil 2. This specified that if a Class 1 emergency did occur, the designated high-speed train used to evacuate NEST would be purged via an automated self-destruct system.
It was this feature which would not only destroy the train, but also finally kill the horribly mutated Dr. William Birkin.