The sort key used for the NixOS bootloader entries. This key determines sorting relative to non-NixOS entries. See also https://uapi-group.org/specifications/specs/boot_loader_specification/#sorting
This option can also be used to control the sorting of NixOS specialisations.
By default, specialisations inherit the sort key of their parent generation and will have the same value for both the sort-key and the version (i.e. the generation number), systemd-boot will therefore sort them based on their file name, meaning that in your boot menu you will have each main generation directly followed by its specialisations sorted alphabetically by their names.
If you want a different ordering for a specialisation, you can override its sort-key which will cause the specialisation to be uncoupled from its parent generation. It will then be sorted by its new sort-key just like any other boot entry.
The sort-key is stored in the generation's bootspec, which means that generations keep their sort-keys even if the original definition of the generation was removed from the NixOS configuration. It also means that updating the sort-key will only affect new generations, while old ones will keep the sort-key that they were originally built with.