On the left side of the window, you have a list of the Devices in your MIDI rig (it might be empty now). You can have up to 119 such devices in the list. Each Device represents a physical MIDI unit. On the Studio Module disk you have a directory called STUDIO.DRV which contains other directories with ready-made so called “drivers”, descriptions of how a certain unit communicates via MIDI, how it organises its memory etc.
Most drivers are made particularly for a certain make and model. But there are also a few special drivers (called List Drivers) and a "generic" MIDI Driver, which can be set up to work with almost any MIDI unit. These are described below and on page 152.