When content-filtering is enabled, ProxySG will perform categorization lookup for each web request against the available (and enabled) databases.

The look-up sequence is given below

Policy (Any Whitelist or Category made in policy file)
Local Database
IWF (Internet Watch Foundation)
BCWF (BlueCoat Web Filter). This also uses DRTR (Web Pulse) if enabled
Third-Party Database (if selected)