The endorsement does not actually provide insurance except on a reimbursement basis. For example, a standard unendorsed Insurance Services Office, Inc. (ISO), commercial auto policy contains a pollution exclusion that would preclude coverage for environmental restitution. In the event the Pollution Liability—Broadened Coverage for Covered Autos—Business Auto and Motor Carrier Coverage Forms (CA 99 48) endorsement is attached to a regulated motor carrier's policy, the pollution exclusion is modified but not totally deleted.

In either event, if the MCS-90 endorsement is attached, the insurer's obligation with respect to such a situation is determined by the MCS-90 endorsement provisions regardless. If the policy would provide such coverage even without that endorsement, the insurer pays the claim and the endorsement effectively has no impact on the issue. However, if the claim would not have been covered under the policy except for the MCS-90 endorsement provisions, the insurer pays the claim and then has the right to seek reimbursement from the insured.