It is same as a script for any attribute. The result of the script will be the value in the attribute. And this value of attribute is taken into the evaluator at 'run-time' (the script is also evaluated on run-time).
Therefore you could define an attribute under the script evaluator . This attribute, in its field calculator, has a VB or Python script. For example let us say the calculated value of the script is 0, and for the evaluator, if 0 means not traversable, then the result will not include that segment into its traversable path.
This script can also make complex calculations from many attributes and give a result that is evaluated. For example, if you calculate the travel time from various attributes like speed limits, slope, background traffic etc and if the result for 'that' segment comes to be 10 minutes, and say in evaluator you had a condition that dont traverse link segments when travel time is more �?� 10minutes, the solver will then avoid this route.