The Commercial Umbrella Liability policy is designed to provide protection against catastrophic liability losses. It generally is written over various primary personal and commercial liability policies. The umbrella policy serves three purposes: it provides excess limits when the limits of underlying liability policies are exhausted by the payment of claims; it drops down and picks up where the underlying policy leaves off when the aggregate limit of the underlying policy in question is exhausted by the payment of claims; and it provides protection against some claims not covered by the underlying policies, subject to the assumption, by the named insured, of a self-insured retention.