OpenSSH: Meaning of cipher with email address @openssh.com


When using OpenSSH server (sshd) we specify allowed ciphers, host key algorithms and MACs. Some of those ciphers have an email address appended, for example aes256-gcm@openssh.com as opposed to aes256-ctr. What is the meaning of that? I checked sshd_config and couldn’t find an explanation.