What is the meaning of a "system user"/"sysuser" term in Plesk? What does it stand for?
For each hosting subscription, Plesk creates a System User - a user account in the Plesk server operating system. Customers obtain their system users' access credentials from their service providers and use these credentials for connecting to the server through FTP, SSH, and so on. In addition, all operations with files and directories in Plesk are performed on behalf of system users. For example, when a customer adds a new file in File Manager, the subscription's system user becomes the owner of the file.
For additional details, visit Plesk Administrator Guide and Getting Started Guide
System users do not have access to Plesk GUI. To allow a user to login to Plesk interface, create a new Additional User in Users menu.
For details, visit Plesk Administrator Guide.