Which version do I need?
That depends on hardware for example if you have a bleeding edge video card it may only work with the latest version of Debian.
If you have very old PC you might need an older version of Debian.
If your restoring an old install of LinuxCNC 2.7 you might want Debian 9.
If you have a Mesa 7i96S you have to use LinuxCNC Uspace 2.9 Febuary 24, 2022 or newer! This means you have to build a DEB package from source and install it. The version that Debian has is older than Febuary 24, 2022. You can’t install LinuxCNC with the Synaptic Package Manager or use a the LinuxCNC Live Build.
LinuxCNC Uspace 2.9 only runs on an OS that has Python 3.x so that means Debian 11 or newer.
A DEB package is an archive containing all the files including the compiled version of the applications, source codes, configuration files, images, and installation commands.