-transition=safe and DIP1000

[Mike Franklin via Digitalmars-d-learn]

I found the following statement in the DIP1000 document 
(https://github.com/dlang/DIPs/blob/master/DIPs/DIP1000.md)

NB: this DIP is out of sync with -transition=safe 
implementation available in dmd and pending a rewrite. Most key 
points still apply though.

What is/was `transition=safe`?  I can't find any documentation on 
it.

And what does "NB" mean?

Thanks,
Mike