A system user is a user that is created alongside and owns an application. These users provide features to the applications and are usually created with Call Queues, Auto Attendants and Conference Bridges. Certain features, like modifying the voicemail message for a Call Queue, require access to the system user.
To view system users:
1. Go to the Users tab inside the domain
2. Uncheck the "Hide System Users" checkbox
3. System users should now be visible. They will have a grey box next to the system user name with the type of system user they are inside of it.