The one-sentence version
A certificate holder gets a piece of paper; an additional insured gets coverage.
Why the confusion exists
Both appear on the same certificate. The holder's name sits in the box at the bottom; the additional insured status is a checkbox and a line in the description of operations that should cite an endorsement form number. People read the holder box and assume it carries rights. It does not, and the certificate says so in the fine print.
What a contract usually wants
A landlord, general contractor or client asking to be "named" wants additional insured status, often on a primary and non-contributory basis and often with a waiver of subrogation. Those are endorsements. If your policy carries blanket versions, every contract that requires them is satisfied automatically and a live certificate can show the status instantly. If not, each one is a scheduled endorsement requested from the carrier.
How CoverMatch handles it
When you add a holder in the wallet and tick additional insured, CoverMatch checks the policy forms for the blanket endorsement. If it is there, the holder's live page shows the status at once. If not, a change request goes to the carrier and the page shows "pending" until the endorsement is issued, so nobody is told they are covered before they are.