What does that mean suppose i have 3 site and 3 vcenter connected in a linked mode so during installtion i need to install SSO in ONE location and during installation on other two servers/sites do i need to point to the first server ?
See these docs:
vSphere 5.5 Documentation Center - vCenter Single Sign-On Deployment Modes
Multiple Single Sign-On instances in different locations: You can install the vCenter Single Sign-On nodes in this deployment in any order. Any node that is installed after the first node can point to any node that is already installed. For example, the third node can point to either the first or second node.
vSphere 5.5 Documentation Center - Joining a Linked Mode Group During and After Installation
On Machine 3, you upgrade to vCenter Server 5.x. After the upgrade, you configure Machine 3 to join either Machine 1 or Machine 2. Machine 1, Machine 2, and Machine 3 are now members of a Linked Mode group.
See this guide on step by step to install MultiSite SSO and Linked Mode: Back To Basics: vCenter 5.5 with MultiSite SSO and Linked Mode Configuration Mike Laverick
Read this blog for the explanation between the three deployment options: Getting ready to upgrade production to vCenter Server 5.5? Make sure you're using the correct deployment option…