Can the documentation please describe "x-sveltekit-invalidated"

[michaelu123]

Describe the bug

I write a PWA and get an error "The FetchEvent for "https://.../__data.json?x-sveltekit-invalidated=1" resulted in a network error response: the promise was rejected." I googled x-sveltekit-invalidated and found nothing except two issues that seem unrelated. Should the sveltekit docs not describe this message somewhere?
Right now, I am quite disappointed that my PWA does not really work offline, as promised.

Reproduction

https://github.com/michaelu123/nk
I start the app with npm run preview, then start ngrok with port 4173, then I start the installed app on a phone, then I disable the network, then I click on a map marker to display more info about the location (all data being in IndexedDB), then I get the error. If I enable the network, display the data, disable the network, and display the data again, it works, for some time. But I want to understand what x-sveltekit-invalidated is about.

Logs

System Info

Android, App installed from result of npm run build, npm run preview, resp. npm run dev

Severity

annoyance

[rChaoz]

I would also like to add that more docs on inner workings in general would be more than welcome. For example, the __data.json suffix is not mentioned anywhere in the docs.