I have seen other post on this topic and they are not correct and or misleading. Is there currently away in Sense 3.1 to hide a sheet in an application so normal users cannot see it? The read privileged would be only for a Root admin, just to keep it simple.
What you want to do is create a rule that grants read access to a sheet where the name = your_sheet_name and user.roles="RootAdmin"
I would be reluctant to implement sheet security as I think it would require constant maintenance as your environment changes and/or sheet names change. I also think people may be reluctant to answer and give you an exact rule as everyone's environments have different security rules implemented.
Something like this may work. You just have to experiment a little. In this example, it grants read access to the Performance Figures sheet only if your role is RootAdmin.