Having seen other commands only output single @ for repo names, I am wondering why dnf history info outputs doubles:
Packages Altered:
Removed abattis-cantarell-fonts-0.301-7.fc36.noarch @@System
For updates, it outputs single @:
Packages Altered:
Install snap-confine-2.56.2-4.fc36.x86_64 @updates
But at the same time dnf info outputs single @ for System:
Repository : @System
When observing carefully you will realize, that System is the only Repository with an @.
Checking with dnf repolist you see non of them having an @.
My conclusion would be: @System refers to the base where is mandatory (like a kernel) and each system has, while the repo is like a tag and can vary by each system.
Reading:
dnf history info output Packages Altered: at Repository/Repo
If you need an official explanation you can make a question on
GitHub - rpm-software-management/dnf: Package manager based on libdnf and libsolv. Replaces YUM.
and refer to this topic.