Developing new code for realXtend server is mostly done by creating new OpenSim region modules so see OpenSim wiki's Getting Started with Region Modules and other developer documentation in OpenSim wiki:
Adding Generic Message packets is the most essential way of extending the server-client protocol.
To be able to receive new type of generic message a new GenericMessageHandler must be added. This can be achieved by calling the AddGenericPacketHandler method in ClientView class. This can be done in a sparate region module or in new clientview class that inherits the current clientview.
Sending a new type of generic message is achieved by calling the SendGenericMessage in clientview class. This also can be done by either inheriting or adding a new region module. In both cases, make sure to check somehow the clients capabilities that it can understand the message. In early versions of ModreX we are using the clients version number to determine it's capabilities, but as we have learned from browser world, this can't be continued in distant future.