On the AS2, a visit can only have one source of traffic. The attribution of this source is done on the first page view of the visit. Priority is given to campaign tracking (xto in the hit), if it exists, then to the referrer. There is no campaign prevalence: therefore a click on a campaign during a visit will not generate a new session, nor overwrite the organic source of the visit.
On the other hand, there is a notion of persistent sources, called prior-attribution, i.e. a visit or a conversion is attributed to the cookied campaign, in the case of a new visit from an organic source under a default window of 30 days (configurable) following the recording of the campaign in cookie.
The cookied campaign can correspond to a last click or first click attribution depending on your settings.
These persistent attributions are materialized via the metric Visits (prior-attribution) which allows you to obtain the number of persistent visits, therefore visits from an organic source attached to a campaign via a persistence cookie.
The notion of persistence or prior-attribution does not apply to visits whose source is already a campaign.
A user comes to your site via a first sponsored links campaign, then, a few days later, comes back via a display campaign: these visits will respectively have as source the sponsored links campaign for the first one and the display campaign for the second one. They will not have a persistent source.
Then, several days later, the user comes back again by searching for your site on Google (organic source). This visit will therefore have as source "Search Engines" and as persistent source:
- the display campaign, in the case of a last click setting, since it is the last campaign clicked.
- the sponsored links campaign, in the case of a first click setting, since it is the first campaign clicked on