Even the names cert.pem, chain.pem, and fullchain.pem aren't any kind of industry standard, but just a choice of this kind that we made when we first wrote Certbot, because we hoped they would help explain to users what the content of the files was. (And we may have made a mistake there, because fullchain.pem is what almost all software should use, but many users instead pick cert.pem and end up serving an incomplete chain.)