A
security group is a Google Group used specifically for controlling access to organizational resources. A security group is created by updating a Google Group to a security group.
Target audiences are groups of people—such as departments or teams—that you can recommend for users to share their items with. They are created from the Directory section of the Admin Console.
They are both used for controlling access but a security group is for resources and settings configuration while a target audience grants access to a group for users to share items they own. I'm able to view the target audience permission in the ACLs for a file and it has the below format: