Search scopes
Search Scopes allow you to partition or organize search collections to enable site users to select different areas to search. So you might want to create all of indexes under the All sources default scope or create specific scopes. The defined scopes are visible to site users from the search dialog.
Whether you should define a specific search collection in its own scope or in an existing scope is entirely up to you. The only difference is whether site users will be able to independently select that collection or not. Site users can only see scopes as shown in the following screen capture.
Parent
Get started with search