YouTube third-party ad vendors
https://support.google.com/3pascertification/table/4572195?rd=1
Vendors that are certified for third-party ads
We conduct a thorough certification process to ensure that ad networks meet Google’s requirements for third-party ad serving with regard to user privacy, accuracy of measurement, latency, and compliance with our creative policies. At this time, we’re focusing on our partners and vendors that have been specifically requested by our publishers.