One of the most common policy endorsement requests is to add an additional insured to a commercial general liability (CGL) policy. Parties who add others as additional insureds are sharing their liability insurance with that other entity, and may have to pay an additional premium in order to do so. These endorsements generally promise to defend the additional insured in the event of a claim or suit.
Typically, the insured who controls the work or the property provides the primary insurance, adding the owner or landlord as an additional insured. For example, tenants add building owners as additional insureds; manufacturers add vendors; and subcontractors add contractors or owners.