An industry consortium or third-party owned exchange that offers a many-to-many model.
Public exchanges are when you trade with public people, meaning anyone can trade with you, if they want to. Public Exchanges are trading venues open to all interested parties (many sellers and many buyers) that use a common technology platform and that are usually run by third parties or industry consortia. Public exchanges support community activities like distributing industry news, sponsoring online discussion groups, blogging and providing research.