As stated in https://www.redhat.com/en/blog/centos-stream-building-innova...
"CentOS Stream now sits between the Fedora Project’s operating system innovation and RHEL’s production stability"
So if we compare to Debian, this means:
Fedora <=> Debian unstable (Sid)
CentOS Stream <=> Debian testing (currently Bullseye)
RHEL <=> Debian stable (currently Buster)
Rawhide <=> Sid
Fedora <=> Semi-stable-ish but short-lived and moves fast.
CentOS Stream <=> Bullseye but possibly less stable than Fedora in parts.
RHEL <=> Buster
FreeBSD 13-Current - Fedora Rawhide/Fedora
FreeBSD 12-Stable - CentOS Stream
FreeBSD 12.2-Release - RHEL
CentOS Stream will be the testing for the next minor release of RHEL, i think the closest in debian would be stable with the proposed repo fully enabled.
Between Debian and RH i see the following equivalency.
unstable (sid) - Fedora Rawhide
testing (bullseye) - Fedora, although not 100% equivalent as not all fedora releases became an RHEL, but fedora does act as testing for RHEL.
stable (buster) - RHEL
CentOS Stream is between testing and stable.