An additional insured endorsement is the policy attachment that legally extends a sub's coverage to the GC. Common forms include CG 20 10 for ongoing operations and CG 20 37 for completed operations.
Requesting this endorsement, rather than trusting the additional insured box on the ACORD 25, is what separates assumed protection from real protection. Many denied claims trace back to a certificate that claimed AI status the policy never actually granted.