A contract can require any insurance it likes. Whether the risk actually moves depends on six pieces of paper most compliance programs never read.
Additional insured
The contract says you're covered under the vendor's policy. The policy says nothing of the kind — unless an endorsement adds you. No endorsement, no status; the contract clause is a promise the policy never made.
Waiver of subrogation
Without it, the vendor's carrier can pay the claim and then sue you to get its money back. The waiver is what makes the transfer permanent instead of a loan.
Primary and noncontributory
Decides whose insurance pays first. Absent this endorsement, the vendor's carrier can insist your own policy share the loss — which means your claims history absorbs a claim you paid to transfer.
It's attached to the policy or it isn't; there's no partially endorsed.
Notice of cancellation
A policy can die mid-project and no one is obligated to tell you. This endorsement creates the obligation. Without it, your certificate file describes coverage that may have lapsed months ago.
Per-project aggregate
A general aggregate limit is shared across everything the vendor does for everyone, all year. If another job burns through it, your project is insured by an empty tank. Per-project aggregate gives your work its own limit.
Completed operations
Most construction and installation losses surface after the work is done — and after the base coverage window closes. This extension is the difference between coverage during the job and coverage when the defect actually shows up.
Each of these is binary. It's attached to the policy or it isn't; there's no partially endorsed. That's why they can be verified as documented facts rather than assessed as opinions — and why a certificate, which shows none of them, can't tell you whether risk transferred or merely appeared to.
Six keystones. An arch missing one doesn't stand a little worse. It doesn't stand.