While I understand third-party cookies, I am not sure to understand what exactly "fourth-party" means in sentences such as:
Third and fourth party ad tracking is allowed on [website] subject to the following guidelines and policies.
Media Company does not have visibility into Agency’s deployment of Fourth-Party Ad Tags
[...] privacy and compliance risks that can come from the use of third and fourth-party ad vendors.
Question: In the web advertisement world, what does "fourth-party" usually refer to?