When visitors convert a Goal, Piwik tries to attribute the conversion to the referrer that brought the visit to the site. Piwik will credit this Goal conversion to the last non empty referrer used by the visitor (ie. the last time the visitor came to the website with a referrer that was not a direct entry). In some cases several referrers can lead a same visitor to your website (eg. Search engines, Newsletter link). Piwik attribution model lets you choose whether to credit the conversion to the first referrer used or the last referrer used.