I could not find any open API in TMF678 - Customer Bill Management for objects of Bill Line Items - like RC,NRC,Usage etc.
Is there a different spec which i need to refer, please suggest.
The line items (charges) in a bill are AppliedCustomerBillingRate, which are indeed dealt with in TMF678 Customer Bill Management. Currently, ACBR is a "read-only" entity, as indeed is the CustomerBill itself. There is a working assumption that charges and the bills themselves are generated by a batch process, not by individual API operation calls for each entity.
Hope it helps