Piwik measures your users and their interactions with your website or app. In order to accurately measure the number of known and active unique users to your app, you can use the User ID feature which will let you assign User ID string to each of your signed-in users. When this User ID is assigned then Piwik will calculate a new metric called “Unique users”. This metric is reported in Visitors > Overview report and will also appear in the
VisitsSummary.get API response under
nb_users metric (more info). This metric represents the number of unique active users (visitors with a known User ID). Learn more in the User ID guide.