What is a System User?
This article explains what a system user and how you can use them.
A System User a type of user that is created by the portal for functionality on certain features.
It is not designed to be used or interacted with and does not count towards the total user count of a domain.
For more information see Understanding The User.
You can view what system users are in a domain by unticking the "Hide System Users" box at the bottom of the User list within the domain view.