A ticket can have many fields, and admins can add new ones. However, the system adds a few fields by default, and admins can only make a limited set of changes to them. These are called system fields.
Follow the steps to check the available system fields:
Log in using your admin account.
Go to Settings->Ticket Fields.
Check fields with the System field badge.
Note: System fields cannot be deleted, nor can their types be changed.