You add a communication module to your main chassis and establish a network (controlnet, ethernet) to connect to other chassis or remote I/O. Since the secondary chassis and remote I/O are declared in the I/O tree of your project, you can manage their modules as if they were installed on the main chassis.
I recommend you to visit Rockwell Automation webpage. Under Product Resources > Literature Lybrary you can download all the literature about ControlLogix platform.