What is the effect of using the 'Not Set' rule option

From Netintelligence Knowledge Base

The Not Set rule option indicates that no action will be taken for this category. When setting up policy rules, we recommend that you use the Not Set rule option in preference to the Allow option. This enables normal internet use (i.e. users can visit sites as they would normally with no action from the Netintelligence Client). The Allow rule option should be used if you want to unblock URLs that are blocked by virtue of being included in a predefined Netintelligence category.