Second and third-party ads, or spam from a source _of_ a source. For example: Someone pays Yahoo! for banner ad space, but then has to withdraw their ad at the last minute. They can’t get a refund from Yahoo!, so instead they sell or sub-lease the ad space they have to another party, or an “affiliate”.
Another way to look at it would be through the over-the-airwaves broadcast TV model. The TV networks send out a signal, or content. That content gets sent out, or repeated, at broadcast stations that are local affiliates, or second parties. Both the network and the station are separate entities in terms of buying airtime for advertising purposes. It gets even more complicated still when a local broadcast station also becomes the go-to station on cable TV as well since the cable company is _also_ a separate party when it comes to ad time. So the network can buy one set of ads, end up having to deal with affiliate broadcasters _already having_ that time in certain locations, and then end up having conflicts with the cable providers wanting a _third_ set of ads in the same place….and this is before the advertisers resell and sublet air-time they’ve bought to each other.
In short terms: It’s not just spam, it’s _complicated_ spam. Good luck figuring out whose fault this _really_ was.