'B' labels are for blockers, not issues which belong elsewhere.
Prompted by this comment on Reddit:
https://www.reddit.com/r/rust/comments/40nzkd/contributing_to_the_rust_compiler_a_short_guide/cyxcjtd
* Yellow, **A**-prefixed labels state which **area** of the project an issue
relates to.
-* Magenta, **B**-prefixed labels identify bugs which **belong** elsewhere.
+* Magenta, **B**-prefixed labels identify bugs which are **blockers**.
* Green, **E**-prefixed labels explain the level of **experience** necessary
to fix the issue.