Sign In With Apple has a "group apps" category: https://help.apple.com/developer-account/?lang=en#/dev04f3e1cfc
However, thats the only information I could find for "sign in with apple" and "group apps".
What is the significants of "group apps" with "sign in with apple"? What does it do? How does one use it?
Does it support the following use-case?: I have a primary app and a child app. If I "sign in with apple" with the primary app, and then i want to use the secondary app, do i have to sign in again? Or am I already signed in because I have signed in with the primary app? Or can I architect it in such a way that allows me to sign in to one of the grouped apps, and then stay signed in when using another grouped app?
From Certificates, Identifiers & Profiles, if you're enabling an App ID for the first time or for a new app, enable the App ID as a primary. You can use primary App IDs on their own or to enable identifiers for related apps and websites through grouping.
To enable an App ID for a related app, (for example, an App ID for the iOS version of your Mac app), group it with the existing primary. This will also ensure that users only need to provide consent to share their information with you once for each group of apps and websites.
Note: This behavior is also outlined in the Developer Account Help document you mentioned previously.