Final answer:
Upon logging into Active Directory on Windows Server 2012 R2, a security token is generated, which identifies the user and their group memberships, governing access to network resources.
Step-by-step explanation:
In Windows Server 2012 R2, after a user logs on to Active Directory, a security token is created that identifies the user and all the user's group memberships. This token is essential for the authorization process, allowing the system to determine what resources the user can access based on their permissions. Group memberships are crucial since they often define the permissions a user has within the network.