An endorsement is the actual policy language that modifies coverage, such as adding the GC as an additional insured or making the sub's policy primary and noncontributory. Unlike a certificate, an endorsement is part of the binding contract between the sub and their carrier.
This distinction trips up many GCs: a COI may check a box claiming additional insured status, but only the endorsement form proves it. Requesting the endorsement, not just the certificate, is the difference between assumed and actual protection.