/usr/src/sys/stand:

This directory is a relic of former directory structure.
The stand-alone programs {boot, tpboot, copy, bootblocks etc.}
have all been removed, as they are now built from /usr/src/sys/boot/.

The PROM is still built from /usr/src/sys/stand/mon/,
and the mon header files there are still accessed
by an install_h from /usr/src/sys/, and by kernel and
stand-alone files via #include<mon/blah.h>.

This directory structure is not pretty, but like
toxic waste, it is a pain to clean up properly.