What is the difference between a customer asking to be named as a certificate holder as opposed to an Additional Insured on my Commercial Insurance Policy?

When a client asks for a certificate of insurance, they are basically asking for proof that you have insurance just as an officer asks for your auto identification card when you get pullover over for speeding. However, when they ask to be named as an additional insured, they are asking to enjoy all the coverages that you enjoy under your policy. So, at time of claim if they are named as an additional insured your policy covers them as well prior to having their own coverage respond, if applicable.