How to modify a DFF so that it only shows for one absence type and is only visible to HR users

Summary:

Content (required):

I have added a descriptive flexfield for Absence Recording to allow HR to select from a dropdown list. However, I ideally only want the DFF to exist for the absence type Sickness Absence, whereas it's currently available for all absences. Is there a way to modify the DFF to achieve this? Additionally, the DFF is showing to Employee/Line Managers when they record an absence but I only want it to be used by HR - is there a way to prevent employees from seeing it (or at least make it read only)?

Thanks in advance for any advice!