Additional insured: the endorsement that actually protects you

The certificate holder is simply the party the certificate was issued to — it confers no rights under the policy. An additional insured, by contrast, is actually covered by the vendor's policy for claims arising out of the vendor's work.

If your organization is only the certificate holder, the vendor's insurer has no obligation to defend or indemnify you. The additional insured endorsement is what changes that.

On a standard ACORD 25 form, the general liability section has a checkbox column for additional insured, and the description box typically names the additional insured parties and the endorsement form numbers (commonly CG 20 10 and CG 20 37 for ongoing and completed operations).

The checkbox alone is not enough — the description should name your organization, and ideally reference the written contract that requires the coverage.

The endorsement applies to ongoing operations only, but your contract also requires completed operations (a real issue for construction work).

The endorsement is scheduled — it only covers the named party, and your entity's legal name is wrong or missing.

The vendor's agent checked the box but the endorsement was never actually attached to the policy. When a claim is large, ask for a copy of the endorsement itself.