To allow UC's customers to log in to CK's site, an Experience Cloud consultant should recommend establishing SSO between the two Experience Cloud sites by using one org as an identity provider and the other org as a service provider. SSO is a feature that allows users to authenticate with one system and access multiple systems without entering their credentials again. To use SSO between two Experience Cloud sites, you need to use one org as an identity provider (IdP), which is the system that verifies the user's identity and issues a security token. You also need to use the other org as a service provider (SP), which is the system that accepts the security token and grants access to its resources.