Umbrella and Excess insurance policies provide additional coverage on top of scheduled primary policies, like General Liability, Commercial Auto, and Workers Comp.
For example, if there is a claim and the primary insurance policy pays out their total insurance limit, any additional amount would be paid by the umbrella or excess policy (up to the limit of the umbrella policy).
The difference between an Umbrella policy and Excess policy is whether or not there is any additional coverage for exposures not insured by the primary policy.
Excess insurance is normally a "follow form" policy that conforms exactly to the coverage of the underlying primary policies, whereas true Umbrella policies have an umbrella coverage feature for additional exposures.
Umbrella policies tend to provide broader coverage over primary policies, in that they usually lack "follow form" clauses, their definitions of what is covered may be broader than the definitions in the primary policies, and they sometimes lack exclusions used in underlying primary policies. Thus, an umbrella policy may cover certain risks which were never covered under the primary policies.