The man.conf(5) man page says

  Manual pages are normally expected to be ... named with a trailing ``.0''.

I see that even old 4.4BSD-Lite's /etc/man.conf has
_suffix         .0

But, I have noticed that HP-UX, most Linux, Tru64 Unix, and other BSDs use
a suffix number that corresponds to the cat directory it is in.

I am curious, why does NetBSD and old BSDs use .0 suffix for preformatted
manual pages?

I am thinking about this, because some pkgsrc packages install
preformatted cat pages with .0 suffix, but man-db (on another OS) doesn't
see or use these .0 files.

   Jeremy C. Reed
   http://bsd.reedmedia.net/