Mozilla are proposing IPA[1] which is designed to track user interaction with ads and product marketing, and track any conversion that occurs (e.g. users end up purchasing something).
If you are shown a product ad whilst browsing searchengine.example and then later look up the product at reviews.example, then end up making a purchase at shop.example, your browser sends all of these events to an aggregation service that allows shop.example to understand (at least in aggregate, assuming you trust the cartel running the aggregation service) that you were exposed to their product at searchengine.example and further exposed to their product at reviews.example.
If you are shown a product ad whilst browsing searchengine.example and then later look up the product at reviews.example, then end up making a purchase at shop.example, your browser sends all of these events to an aggregation service that allows shop.example to understand (at least in aggregate, assuming you trust the cartel running the aggregation service) that you were exposed to their product at searchengine.example and further exposed to their product at reviews.example.
[1] https://github.com/patcg-individual-drafts/ipa/