When you create the TXDataClient object, it uses the default entity model to retrieve the available entity types and service operations that can be retrieved/invoked from the server. When your server has multiple models, though, you need to specify the model you are using when accessing the server. This is useful for the client to know which service interface contracts it can invoke, and of course, the classes of entities it can retrieve from the server. To do that, you pass the instance of the desired model to the client constructor:
Client := TXDataClient.Create(TXDataAureliusModel.Get('Security'));
See topic "Multiple servers and models" for more information.