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The database user for MongoDB is initially created by the BVQ installer during installation. You can change/delete existing user or create new users if needed. See LINK_TO_MONGODB_USERSee Database users in MongoDB
For BVQ by the default there is only 1 db user necessary. 
If the password for the DB user is lost, there is no way to access the database. Keep the password save.
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These users are needed to log into the Web UI. There are different roles with different privileges available.
See LINK_TO_WEBUI_USERSee Web UI users
By default there is one bvq user created during the installation. This user has administrative rights within the WebUI. By default this user has the same credentials as the Database user in MongoDB.
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Grafana has its own user management, therefor a separate user is created within grafana during the installation of BVQ. Since version 5.4.x this is an administrative user.
See LINK_TO_GRAFANA_USERSee Grafana users
During installation a second user called admin is created. This user is used BVQ internally only.