There's no hierarchy to the schematic. Pages are inserted/deleted in a freeform way, the same as you'd do with a Word document. Reference designators are in a global namespace, and so are net names.
It feels like it's a pretty common drafting style, at least in my corner of the world. I've worked at two companies that use this style, and I also see it a fair amount on reference-designs like the linked example.
When I need this sort of schematic in Kicad I have the top-level be a "table of contents" page and everything else is below that. Works great, you can insert/delete pages at will and also you have a nice place to put docs about what is on which page. You then need to use global labels instead of hierarchical labels to link stuff together of course.
KiCad's multi-sheet schematic is hierarchical. There is one root sheet and beyond that the structure is flexible.
Do you have an example multi-sheet schematic as you'd like to be able to have in KiCad?