This option compares the number of Flow monitor sources allowed by your WhatsUp Gold license against the number of sources which have are sending traffic to WhatsUp. If you reach the source limit allowed by your Flow monitor license, you will not be able to receive traffic from any additional sources.
You can either purchase an upgrade to a license with a higher source limit, or disable un-used sources in the Flow Monitor Source Configuration dialog
Licensing and support information is available on the WhatsUp Customer Portal. The web portal provides enhanced web-based capabilities to view and manage Ipswitch product licenses.