This is an important NixOS Flake example. Why? Because it contains the following code/technique: # You can also use a specific git commit hash to lock the ve

This is an important NixOS Flake example. Why? Because it contains the following code/technique: # You can also use a specific git commit hash to lock the version nixpkgs-fd40cef8d.url = "github:nixos/nixpkgs/fd40cef8d797670e203a27a91e4b8e6decf0b90c"; Why is that important? Because (as opposed to a construct like: nixpkgs.url = "github:nixos/nixpkgs/nixos-23.05"; OR nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable" ) -- nixpkgs-fd40cef8d.url truly is fixed/pinned/unchangeable/static/"set in stone"/"guaranteed to contain exactly the same code at every point in time"/"locked" (use whatever terminology you will). Whereas nixos-unstable is always changing, and nixos-23.05, while reasonably stable -- is not guaranteed to be exactly the same code in the future (bug fixes and patches may be applied to it retroactively). But, "github:nixos/nixpkgs/fd40cef8d797670e203a27a91e4b8e6decf0b90c" is always guaranteed (by means of the commit hash) to be exactly the same code, perpetually and always, into the future, continually... So, it's a great technique for software hermeticism and reproducibility and container deployments / build recipes. (In the Flake example above, we also see nixpkgs/nixos-23.05 and nixpkgs/nixos-unstable used as additional Flake inputs. NixOS Flakes support this -- but if hermeticism and reproducability are desired, then only a single nixpkgs.url tied to a specific commit hash via the URL technique shown above, would be used...)