OnixS C++ CME DropCopy Handler version 4.0.0 includes several important changes.
The full list of changes available here.
In versions prior to DropCopy 4.0 there is Application Message Resend logic which was used to request missed messages. From version 4.0 CME removed this functionality, so only FIX resend can be used to receive corresponding FIX messages. FIX resend is done automatically by OnixS FIX Engine. For more information please see Resending Messages.