Here is the detailed description on SSO multisite and vCenter linked mode:
Introduction to Multisite vCenter Single Sign-On Deployments
The vCenter Single Sign-On multisite configuration is designed for deployments with multiple physical locations. Installing a vCenter Single Sign-On instance at each site allows fast access to local authentication-related services. Each vCenter Single Sign-On instance is connected to the local instances of the AD (LDAP) servers and has its own database with local users and groups.
Multisite deployment is useful when a single administrator needs to administer vCenter Server instances that are deployed on geographically dispersed sites. To view all vCenter Server instances from a single vSphere Client or Web Client, you must configure the vCenter Server instances in Linked Mode.
Note: Multisite Single Sign-On deployment is designed only for faster local access to authentication-related services. It does not provide failover between Single Sign-On servers on different sites. When the Single Sign-On instance on one site fails, its role is not taken over by a peer Single Sign-On instance on another site. All authentication requests on the failed site will fail, even if peer sites are fully functional.
In multisite Single Sign-On deployments, each site is represented by one Single Sign-On instance: one Single Sign-On server, or a high-availability cluster. The Single Sign-On site entry point is the machine that other sites communicate with. This is the only machine that needs to be visible from the other sites. In a clustered deployment, the entry point of the site is the machine where the load balancer is installed.
Note: For further information, please see:
vCenter Single Sign-On Deployment Modes
You can join multiple vCenter Server systems using vCenter Linked Mode to allow them to share information. When a server is connected to other vCenter Serversystems using Linked Mode, you can connect to that vCenter Server system and view and manage the inventories of the linked vCenter Server systems.
Linked Mode uses Microsoft Active Directory Application Mode (ADAM) to store and synchronize data across multiple vCenter Server systems. ADAM is installed as part of vCenter Server installation. Each ADAM instance stores data from the vCenter Server systems in the group, including information about roles and licenses. This information is replicated across all of the ADAM instances in the connected group to keep them in sync.
When vCenter Server systems are connected in Linked Mode, you can perform the following actions:
You cannot migrate hosts or virtual machines between vCenter Server systems connected in Linked Mode.
Refer:vSphere 5.5 Documentation Center
Additional resources:
vCenter Single Sign-On - Part 2: Deployment Options | VMware vSphere Blog - VMware Blogs
vSphere 5.5 - SSO Multi-Site Deployment - Learning by Doing | vTricks.com