An excess, or secondary, insurance policy extends the coverage limit of the primary policy. The primary policy will be used first to cover a claim. The excess policy would then be used to cover the same claim up to a higher limit.
What is the difference between excess and umbrella policies?
An excess policy adds extra coverage for losses already included in a primary policy.
An umbrella policy adds coverage to your primary policy but also may cover losses or risks not covered in another policy.