build dependencies vs tools

[Jan Danielsson <jan.m.danielsson%gmail.com@localhost>]

Hello,

   Looking at some dependencies on pkgsrc.se and a thought occurred:
What's the difference between "build dependencies" and "USE_TOOLS"?

   Say a project needs a bison-like tool, but not actually bison, to
build; it can be added to the list of "tools" or it can be made a
"plain" (for lack of a better term) build dependency.  Ignoring that one
can simply make nonexistent "tools" into build dependencies on the fly:
How is something qualified to be a "tool"?

   Are "tools" merely "very common build-related programs which are very
likely to exist (and be usable to pkgsrc) in the system already"?

   (Just curious, not looking to upend anything).

-- 
Kind Regards,
Jan