A Sunset Clause is an endorsement to a General Liability policy that states the policy will not provide coverage after a set period of time. What this means is that the carrier will not even respond to determine if the occurrence resulting in a claim could be covered by the policy if the claim is made after the sunset.
This is in sharp contrast to a regular Occurrence Form policy which has no limitation as to when a claim can be made. Whether or not coverage would apply is a separate issue but without a Sunset Clause the carrier would appoint an adjuster to determine if coverage applies. With a Sunset Clause no action would be taken if the claim were reported after the sunset date.