Every Teamup Calendar has default event fields and custom fields can be added.
In the event editor, all event fields are shown:
Read-only users do not see the event editor since they do not have permission to modify (edit) events. When read-only users open an event, they see the event viewer. The event viewer only shows event fields with contents.
Empty fields are not shown in read-only view. So in the event above, the event viewer does not show the Who field, the Description field, the Notes field, or the Attachments field because these fields are all empty.
The Comments field will be shown if event comments are enabled for all users.
Note that if a field's visibility is set to Users with modify access only, then the field will not be visible in the event viewer. It will be remain hidden from read-only users with or without data in the field.