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.
Want to know the true value of all your marketing channels including all individual referrers? Piwik lets you apply attribution models using the Multi Channel Conversion Attribution premium feature. Looking at only the first or the last referrer usually doesn’t let you know which channel is actually the most successful. Why? When a user visits your website several times, you only look at either the first or the last channel. However, you don’t give any credit to all other referrers the same user has used to visit your website. With the multi attribution feature, you can for example choose to give each referrer the same weight or to give all credit to the last non-direct referrer.