Gregory's ServerNix is a package manager, which installs all software to a big library called the Nix Store, at /nix/store. The closure is a package and every other package it needs. So the receipt is me going to the Nix store, getting a single thing, and seeing the giant list of everything else it depends on :)
(In this case everything to setup my Framework laptop)
That and receipt printers are so much more fun to own than anyone would expect