A new user discovers a website for the first time by clicking a sponsored link on a search engine results page. One week later, the same user returns to the site by clicking a link from an email campaign. In a report analyzing this second session, what will the dimension First user source display?

The first user dimension reports Paid Search. 'First user' records how a person was originally acquired, and it never changes for that user afterwards, no matter how many times they come back or through which channel. Here the first visit came from a sponsored search result, so Paid Search is permanently recorded as the acquisition channel. The later email visit is captured too, but in session-scoped dimensions, which describe the source of each individual session. This distinction is the backbone of several exam questions and of sensible reporting. First user dimensions answer 'which channels bring new people to us', while session dimensions answer 'which channels bring visits', and attribution-scoped dimensions answer 'which channels get credit for conversions'. Reading one and interpreting it as another is how teams end up crediting email with acquisition it did not do.