It doesn't seem to be defined in the standard, but it's partially documented in Chromium's docs how they determine what is an ad resource:

https://chromium.googlesource.com/chromium/src/+/master/docs...

https://chromium.googlesource.com/chromium/src.git/+/master/...

https://chromium.googlesource.com/chromium/src.git/+/master/...

This seems to indicate the authoritative source of truth is EasyList. On my current machine, the list seems to be stored in "~/.config/google-chrome/Subresource Filter/Unindexed\ Rules/9.22.0" and should be easily inspectable.

I don't know if I've missed some documentation pointers related to this.