What does primary and noncontributory wording mean?
It means your policy pays first for the additional insured and does not ask the additional insured's own insurer to share the loss. The ISO form that delivers it is CG 20 01. Its operative sentence: "This insurance is primary to and will not seek contribution from any other insurance available to an additional insured under your policy."
CG 20 01 attaches two conditions. The additional insured must be "a Named Insured under such other insurance," and you must have "agreed in writing in a contract or agreement" that your insurance would be primary and noncontributory. If the GC's own policy is in a different entity's name, or there is no signed contract, the condition fails. The ACORD 25 has no box for it; it appears only in the description of operations, and only accurately if the endorsement is on the policy.