Matomo (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 Matomo will calculate a new metric called “Users” which is the number of unique users who used your app in the selected time period. 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. This metric is enabled only for some periods, and this can be configured (FAQ).