What is the file to be specified in guix system reconfigure?

The [documentation](https://guix.gnu.org/manual/en/html_node/Invoking-guix-system.html) for guix system reconfigure says: > Build the operating system described in *file*, activate it, and switch to it. A cursory search didn't bring up any information on what or where that *file* is or how to generate it. As @muru commented, that page also says: > Once you have written an operating system declaration as seen in the previous section ...but that section is [Bootloader Configuration](https://guix.gnu.org/manual/en/html_node/Bootloader-Configuration.html) and likewise does not tell me where to find that file on my system. Since this step is part of a normal system upgrade (after runnnig guix pull as described in multiple places), I would've expected something a bit more obvious... but since I am a complete noob in GuixSD, I could easily be wrong.