Explanation of Export Forest option for Active Directory deployments
Includes all domains for which the user defined in LDAP Connection settings is a member. Selecting Export Forest will automatically select the root forest domain and enable that domain. All other domains will not be visible. When Export Forest is selected all domain object queries are done using Global Catalog. The Top Level Node is not editable when using this option.
Checking the box for any domain enables that domain for export, Top Level Node and Filters become editable for that domain. Selecting the row will display the top level node and filters for that domain.
The Top Level Node filters is the DN path that will filter all objects from that path downwards. For example, a top level node might be: