Comment from Chris: This line means that only domains configured (in portlet properties) as active are provided to the user. Allowing for domains to be 'turned' off/on universally after deployment.

Response:  This addressed what was a confusing situation to me.  When I added LdapDomain to domains.xml the end user would see "Online Directory" in the edit portlet config UI.  However after checking the checkbox to include it, the Online Directory did not show up on the portlet view because it was not enabled in the portlet's configuration (from the admin manage portlets).  I thought the portlet's edit config UI shown to users should only display those items that are available to display to the user.