Retargeting
Retargeting is the advertising practice of showing ads to people who previously visited a website or used an app, based on identifiers collected by a pixel or cookie during that visit.A retargeting campaign starts on the advertiser's own site. A tag such as the Meta Pixel or the Google Ads remarketing tag records that a browser viewed a product or left a cart, adds it to an audience, and the advertising platform then shows that browser related ads on other sites and apps. Customer lists uploaded as hashed emails extend the same idea to people who are logged in to the platform.
Because it depends on following a visitor from one site to others, retargeting is the textbook case for the strictest consent rules. In the EU and the UK, the tag stores or reads an identifier and serves advertising, so it needs prior consent under Article 5(3) of the ePrivacy Directive, and the marketing category on the banner must be off until accepted. Google ties remarketing for EEA and UK users to the ad_personalization and ad_user_data signals of Consent Mode v2.
In California, retargeting is the definition of cross-context behavioral advertising in Cal. Civ. Code 1798.140(k): targeting ads based on a consumer's activity across businesses, websites, apps or services other than the one they intentionally interact with. Disclosing data for it is sharing, which consumers can opt out of. The pixel therefore has to stop for a visitor who opts out or whose browser sends Global Privacy Control. See marketing cookies for the category it belongs to.