The second interaction uses the push model. Either of the interactions could use a different single sign-on model. Web user authenticates with security service. Web user requests resource from first destination Web site, providing authentication reference. First destination Web site requests authentication document from security service, passing the Web user's authentication reference.
Security service provides authentication document to first destination Web site.
First destination Web site provides resource to Web user. Web user requests link to second destination Web site from first destination Web site. First destination Web site requests access authorization from second destination Web site, providing third-party security service authentication document for user. Second destination Web site provides access authorization. First destination Web site provides authorization reference to Web user.