Full Answer

Safari's logic is that a parameter appended by one site to identify a user on another site is exactly the cross-site tracking its Intelligent Tracking Prevention exists to stop. Click identifiers fit that definition, so Safari treats them as link decoration and removes them when a visitor crosses from the ad domain to yours. The cookie that would normally store the click is also short-lived under ITP, so even a captured ID rarely survives a multi-day purchase journey.

In place of deterministic tracking, Apple offers Private Click Measurement: the browser itself records that a click led to a conversion, then sends a delayed, aggregated, noise-added report with no individual user attached. It answers "did this campaign work" but never "which person bought", which is the granularity ad platforms need for optimisation and which most attribution models assume they have.

The consequence for store owners is systematic under-credit to paid social and search on Safari traffic, pushing budget toward channels that merely report better. Recovering it means capturing the click identifier server-side at the first request, before Safari's stripping applies, and forwarding it through a conversions API. That moves attribution off the browser entirely, which is the only layer Safari doesn't get to edit.