There are several options for doing this depending on what you want to categorise.
Referral Reason or Referral Source fields could be used to categorise things such as ATAPS referrals, Better Access referrals, Workcover referrals or Private referrals. This is a dropdown list that can be customised.
The other option maybe to re-use a field you are not using for this purpose e.g Benefit Type or Marital Status. These fields can also have their options customised.