A certificate holder is simply the entity that holds the coverage as described on the certificate. For example, you might be a certificate holder for liability insurance, while your landlord is an additional insured because he/she owns the building where you work.
An additional insured is another business that's also listed on your certificate holder's insurance policy, such as a subcontractor. If you're working on a project and your employer hires a general contractor, that person could be both a certificate holder and an additional insured.A certificate holder is simply the entity that holds the coverage as described on the certificate. In most cases, it's the owner or operator of the property. For example, if you're running a construction site, the person signing the contract to build the building is the certificate holder. If you hire subcontractors to do work on the project, those people are the certificate holders.
An additional insured is another business that's also listed on your certificate holder's insurance policy, like a general contractor or supplier. So, if you're working on a large construction project, the general contractor might carry insurance for the entire project. But if you want to add a subcontractor to the list of covered entities, you'll need to purchase separate insurance for each one.