Rich Hutnik wrote on Sat, Nov 1, 2008 08:13 PM EDT:
Proprietary means that the designers, Harper and Seirawan, haven't given
anyone permission to use the pieces for any other purposes than what they
intended. You can't take the Hawk, and use it as something else, and you
are not permitted to use it in any other game, but what Harper and Seirawan
approved of, which now is just Bughouse.
Ok, people on their own can use them as they choose. But, if you wanted
to run a publicized tournament and use their pieces for other than what
they intended, you may risk them suing you. This is what is meant by
proprietary.