There could be trusted websites of partners or vendors whose webmail or file downloads might otherwise be blocked because of TMWS policies. Adding these websites to the Approved URLs List will allow files from them to bypass these policies.
You can also have TMWS block certain web pages, domains, and URLs. URL blocking is not policy based—it affects everyone in the organization. One especially powerful use for URL blocking is to combat a sudden Internet threat such as a DoS attack or a new Internet worm. You can quickly create a rule to stop the threat from spreading or hijacking client machines.
The approved and blocked URLs lists are used as a first check in front of the whole scanning process.
The approved and blocked URLs lists allow you to override the URL Categories and Web Reputation settings configured in cloud access rules.
The approved URLs list takes precedence over the blocked URLs list.