If your insured is a retailer and the manufacturer or distributor wants to be named as an additional insured on your insured's policy, you should be using either the additional insured designated person or organization endorsement, or a manuscripted endorsement that will satisfy the intentions of the parties involved.

The additional insured designated person or organization endorsement from ISO is CG 20 26, and it amends the who is an insured clause in the CGL form to include the person or organization shown in the declarations as an additional insured for liability for bodily injury, property damage, or personal and advertising injury caused in whole or in part by the named insured's acts or omissions in the performance of ongoing operations, or in connection with the premises owned or rented by the named insured. So, CG 20 26 requires the additional insured to be listed on the endorsement and is applied to ongoing operations and premises liability, not completed operations.