When you mark a send connector as scoped, this means it can only be used by Exchange 2007/2010 hub transport or Exchange 2013 mailbox servers in the same Active Directory site as the send connector. If not selected, the connector can be used by all transport servers in the Exchange environment.
How do you determine what servers are in the Active Directory Site? A Send Connector is not bound to any specific Active Directory site Clint, they are merely an object in Active Directory!
True, however say you have an Active Directory site called "Contoso HQ" and the site has 4 transport servers whether they be Exchange 2010 Hub Transport servers or Exchange 2013 mailbox servers. Two of these servers are marked as source servers a of the scoped send connector and two aren't. The two servers which are a member as a source server of the send connector can obviously use the send connector. The other two transport servers in the same Active Directory site which are not source servers can also use the scoped send connector, all other transport servers in other sites - bad luck!
Unmarking it as scoped means transport servers in other sites can relay through the send connector if there is no closer option for external mail relay.
Hope this blog post brings clarification to this topic.