Introduction to Debian
To enter Debian's Stable, sufficient testing must verify that there are no serious problems.
For example, singularity-container for HPC has not progressed to the next stage from the Sid repository.
The experimental repository
This directory contains packages and tools which are still being developed, and are still in the alpha testing stage. Users shouldn't be using packages from here, because they can be dangerous and harmful even for the most experienced people." I think use this repository from the Sid distribution.
https://wiki.debian.org/DebianExperimental
DebianUnstable (Sid, Rolling Release)
Debian Unstable (also known by its codename "Sid") is not strictly a release, but rather a rolling development version of the Debian distribution containing the latest packages that have been introduced into Debian.
https://wiki.debian.org/DebianUnstable
DebianTesting (Trixie, Rolling Release)
Debian testing is the current development state of the next stable Debian distribution. It is also made available under the code name of the next stable release, Trixie is the current testing codename.
https://wiki.debian.org/DebianTesting
Debian Stable (Debian 12, "bookworm")
The "stable" distribution contains the latest officially released distribution of Debian.
Debian Stable is not released until critical bugs are fixed.
https://www.debian.org/releases/index.en.html
DebianOldStable
Oldstable is a codename for the previous Debian stable repository, as long as security updates are provided. It is not just a name of the old stable version(s) of Debian.
https://wiki.debian.org/DebianOldStable
Bullseye (Debian 11) is the current oldstable.