mandatory fields

Mandatory fields are also called as KEY FLEXFIELDS.  Key flexfileds are used to capture mandatory or key business information of the organization.  Key flexfields are DATE, CURRENCY, TYPE OF ACCOUNT.

There is no such rule that you need to have mandatory fields on a DFF.  It works perfectly even without mandatory fields.  But, it is a general practice that we will have atleast one mandatory field in the DFF.

If you want to make any of the fields mandatory, you need to check the "Required" checkbox on the segment:
  Segments Summary form --> highlight the segment --> click on 'Open' button

This opens the 'Segments' form, where you have to check the 'Required' checkbox.