Nixpkgs versioning is designed to be used with the nix manager.

NixOS releases follow the `YY.MM` pattern.
There are also some prefixes and suffixes allowed.
Using `22.05` as an example the allowed combination of prefixes and suffixes are:

- `release-22.05`
- `nixos-22.05`
- `nixos-22.05-small`
- `nixos-22.05-aarch64`
- `nixpkgs-22.05-darwin`

Additionally, there are some floating versions:

- `nixos-unstable`
- `nixos-unstable-small`
- `nixpkgs-unstable`