As already mentioned by our other colleagues, all CRM online related user exists is valid of CIC0 also.
For example, if you are interested in BP transfer to R/3, you need to write user exists for CRM classifications whihc can be found in SPRO ->Cross application -> SAP business partner --> business partner --> field grouping.
Just an example.
As you must be aware, CIC0 is just a front-end application. For example, if you create a sales order in CIC0, normal order creation process of CRM Online will be followed. So, all normal CRM Online order related user exits will be valid for CIC0 as well.
Similarly for other area like activity & so on...
Hope this helps.