What feature is used to track unsupported ad types that specify their own landing page or Google Display Network and Gmail promoted campaigns?
Landing page test
The correct answer is:
A placeholder keyword is a keyword in a search engine account or “other” engines account that provides Search Ads 360 with the information it needs to track visits. For example, you can use a placeholder keyword to report on placements on the Google Display Network, or report on ads in engines that Search Ads 360 doesn’t support.
Read more here: https://support.google.com/searchads/answer/6270161