WTF is a Type B Confined Space?

While dequehead was joining the front of the queue this morning he noticed this fine warningsemble on an otherwise uninteresting pair of doors:

So let's break it down and see what we can see.

So what is SF6 then? Oh, right. Well that's clear enough. What else do we have...

This one is great. Not a particularly tough looking padlock. But if you do... you might die. Got it. What else?

Gosh, look at this monster. Its all about these warning signs. This is a complex subject, worthy of further study. The interesting thing, to dequehead, is that in his research around the subject he never did find a definition of a Type B Confined Space.