Shared roster is configuration setting when XMPP server initiates modification of client’s roster. Having connected to a server a client receives a list of groups and contacts within groups. Apart from contact information the client automatically receives subscription from all contacts and subscribes to them as well. Thus every user on the roster is aware of status of any other contact.
In “Roster” tab you can see list of all contacts in all jabber-domains set up on your ICS CUBE. The view is identical to the way they are displayed in a contact list of a user connected to the ICS CUBE via his jabber account.