It means there are two separate chemicals inside of the building. Each chemical would have their individual labels on their containers, but these external signs are for first responders who haven’t entered the building yet.

Let’s say there’s a fire. The red diamond tells them how likely it is that the chemicals are the cause of the fire, and where they should direct their efforts. The blue corner tells them what kind of PPE they need to use if they enter. The yellow tells them what kind of potential explosive risks the chemicals have. And the white one is especially important, because the W means the chemical reacts with water; If there’s a fire at the facility, they can’t simply use fire hoses to fight it.

The reason for listing them separately is because each individual chemical has its own ratings. You can’t simply take the highest of each and combine them into a single sign. For instance, in this case one chemical isn’t flammable but is explosive when heated. The other chemical is flammable but not explosive. So if you see a chemical on fire, you know it’s the second chemical and isn’t explosive. But if you see something that isn’t burning in a room full of fire, you know it’s a potential powder keg waiting to explode.