These three terms show up on nearly every subcontract's insurance requirements, and they're often requested together because each one closes a different gap.
Additional insured (AI)
An endorsement added to the subcontractor's liability policy that extends some of its coverage to the general contractor for claims arising out of the sub's work. Not all AI endorsements are equal — some cover only "ongoing operations" (while the work is in progress), while broader forms also cover "completed operations" (claims that surface after the job is finished, which matters a great deal for construction defect claims that can take years to appear).
Waiver of subrogation (WOS)
After an insurer pays out a claim, it normally has the right to "subrogate" — to pursue whoever else may have contributed to the loss and recover what it paid. A waiver of subrogation means the sub's insurer gives up that right against the GC, even if the GC was partly at fault. Without it, a GC can end up sued by its own subcontractor's insurance carrier after a claim that seemed fully resolved.
Primary & non-contributory (P&NC)
When both the sub's and the GC's insurance policies could respond to the same claim, the default rule in many policies is that both pay a share, split proportionally. Primary & non-contributory changes that: the sub's policy pays first (primary), and the GC's own insurance isn't drawn into sharing the loss (non-contributory) as long as the sub's policy has coverage available.
Why GCs ask for all three
Each closes a different gap: additional insured status gets the GC covered under the sub's policy in the first place; primary & non-contributory makes sure that coverage responds first, protecting the GC's own loss history and premiums; and waiver of subrogation prevents the sub's insurer from coming back at the GC after paying out. Asking for only one or two of the three leaves a real hole even when the paperwork looks complete.