A certificate holder is simply the party the COI is addressed to — the person or company receiving the document. It does not extend any coverage.
An additional insured (AI) is added to the underlying policy via endorsement. When an AI is named, the policy responds to claims against them arising from the named insured's work.
The typical GC-to-sub requirement is: 'Additional insured for ongoing operations AND completed operations, on a primary and non-contributory basis, with waiver of subrogation.' Each of those phrases matters and each requires a specific endorsement.
Blanket AI endorsements (CG 20 33, CG 20 38) are cheaper and simpler than scheduled ones for contractors with many customers. Verify the wording matches contract requirements.
If you sign a contract that requires AI and your policy doesn't have the right endorsement, your carrier can deny defense costs on that party's claim — and you eat them.