difference between sources.list and sources.list.distUpgrade [duplicate]

On ubuntu, in the /etc/apt/, there are two files: sources.list and sources.list.distUpgrade. But I only know they are used to update system. However, what are the the difference between them? by the way, what the GPG keys used for? when I update the sources.list using ubuntu source list generator, should I update GPG keys as well?

Thanks !

When you upgrade between distro versions, .list files in /etc/apt/sources.list.d get commented out. The original versions of those files get backed up with the .distUpgrade extension, and that's what you're seeing. Other than sitting there and being backups, I don't think the .distUpgrade files do anything.

I guess Ubuntu (Debian?) does this out of the presumption that .list files will be incompatible across distro versions, but as a packager, this is pretty inconvenient. Packages like google-chrome install a special cronjob just to work around this issue and un-comment-out their .list file after a dist upgrade.