S_FALSE or E_aaa

One of our exposed COM method returns E_aaa_1 and E_aaa_2 on two different cases, but COM consumer complained this behavior and suggested to using S_FALSE. It is OK to me, but our requirement document specifically claimed that we shall return error code. I want to keep current status and try to identify it in API document, oh I faint, the API also stated that the method would return S_FALSE in case of …