System users are SSH/SFTP users that ServerPilot creates on your server. Every app belongs to and runs as one of these system users. You can log in to your server as a system user to deploy an app or view an app's log files.
By default, ServerPilot creates the serverpilot system user. However, there are benefits to having multiple system users. The ability to create additional system users is a feature of our paid plans.
Multiple system users can provide separate developers access to individual apps on your server and create isolation among apps. If you think you will have multiple developers running separate apps on your server, you should create the system users first as you can't change which system user an app runs under once you have it set up.