The Exceptions tab allows you to create a boolean expression that is tested against each row of the Must Contain and Must Not Contain rules. If the Exception expression evaluates to be true for a particular rule, that rule is ignored. The intent of the Exceptions tab is that you can create an exception expression based on variables and cause certain rules to be ignored. For example, you could create a list of BGP neighbor expressions including all your BGP routers in the Must Contain, and for each particular Device being tested write an Exception expression that caused the neighbor statement in the Must Contain rules that was for the router being tested to be ignored.
Most tests will not require use of the Exceptions tab.