An insurance policy is a contract. The insurer is on one side. Who is on the other?
“Named insureds” are those who own the policy. They are listed on the declarations page.
Other “insureds” are those who are insured on the policy, but not the named insured. These people/entities are usually not specifically named. Rather, they fall within a category of insureds that is created based on the language of the insurance policy. For example, a store manager is probably not specifically named on the store’s insurance policy. But, she is likely an insured under the store’s policy for acts that she does in the course of her work as the store manager.
Additional insureds are added by endorsements. For example, general contractors are frequently AIs on their subcontractors’ policies.
Sometimes an endorsement will name a specific person/entity as an AI. Other times the endorsement will add any party who meet certain requirements.
Coverage for an AI cannot be broader than for the named insured. But, it is frequently limited. Check the language to be sure.