SIGs (Special Interest Groups) are groups within the CentOS community focusing on a specific set of issues. For example, there's the Storage SIG, Virtualization SIG, Infrastructure SIG, Core SIG, Automotive SIG, and the Hyperscale SIG. Each SIG releases its own repositories which contain packages related to a chosen focus.

Once upon a time, users would have to locate a specific repository and then download/install the relevant centos-release-* packages just to enable that particular repository. AlmaLinux decided to simplify that process by releasing upstream CentOS Special Interest Groups' release packages. 

Even at this early release stage, you can already enable SIGs for Cloud, Config Management, Messaging, Network Functions Virtualization, Ops Tools, Storage, and Virtualization. To find out how to enable these SIGs, head over to the AlmaLinux CentOS SIGs Repository page.

As to why AlmaLinux went this route, they've said, "By encouraging developers/contributors interested in the various SIG development to work upstream and submit contributions there (and submit bugs), we can ensure this work remains centralized and continues to be part of the long-term stability, success, health, and vitality of the community."

For SIG developers, AlmaLinux has this to say, "Instead of building against CentOS 8 and CentOS Stream 8, SIGs should build against RHEL 8 and CentOS Stream 8! This would ensure that the SIG's output is consumable by the entire RHEL family of distributions, including rebuilds like AlmaLinux."