You can target all and only all criteria at once so only people who fall into all criteria will see the ad. A variation may target a different group and have it

You can target all and only all criteria at once so only people who fall into all criteria will see the ad. A variation may target a different group and have its own tracking. If they see the ad they fall into the category combination I specify. If they click the ad then I known all of that stuff about a visitor because I know which visitors clicked and which found their way outside of ads. If they signup or enter email or anything like that then I know exactly who they are for most people. Even if I have only an email but no name I can create a shell profile and use a data broker to find the name behind the email a lot of the time. Then I have everything tied to a person. Multiply this times thousands upon thousands of ad impressions across multiple campaigns and you can bud a pretty good database with highly specific data. Sure 100% accuracy isn't guaranteed but neither is it guaranteed through more direct data brokers.