Need to Capture extra Information on a particular benefit plan enrollment results

Summary

I need to provide an extra field for self service benefit enrollments. Every employee that chooses dental plan needs to fill in this extra information field.

Content

Hi

I have a requirement  to capture extra information, when employee tries to enroll into dental plan only. This field needs to be enabled only for dental plan, and should not show up on any other plans like (Medical, savings etc).

Under Manage Descriptive flexfield, I have seen "Enrollment Results Attributes" DFF, can someone please give steps on how to configure this DFF, so that I can see this field only on dental Plan. How to set context that only dental plan shows this extra field.