Message Bus Drivers
The Message Bus is implemented in two drivers, Client and Server. They are DLLs which completely separates the communication methods from application logic. Truxton doesn't know how the message bus is actually implemented, all it knows are how to call functions in an interface to do a job.
Plugin
There are no compile-time linkages to the message bus. The plugins are DLLs that expose an entry-point function by name. This entry-point function accepts a data structure that will be filled with pointers to functions to do all of the tasks in the interface.
Client
The Message Bus Client is for use by applications that need to communicate with message queues. The client implements the MESSAGE_BUS_PROVIDER interface.
Server
The Message Bus Server is for use by applications that need to create or destroy a message server. It cannot send or receive messages. The server implements the MESSAGE_BUS_SERVER interface.