[yocto] What's the purpose of "BDIR" in oe-buildenv-internal?

[Robert P. J. Day]

   prepping to give a yocto talk to my local (ottawa) LUG next week, and
was reviewing the logic of the setup starting with oe-init-build-env,
and noticed in scripts/oe-buildenv-internal a test of the variable
BDIR for the name of the build directory.  but there's no mention of
that variable in the calling oe-init-build-env, and i see no mention
of it in the intro-level docs.

   clearly, one could use that shell env variable if one wanted to name
the build directory to be used, but given that there's absolutely no
explanation of it anywhere for a new user, what exactly is its purpose?

rday