The communication system is a combination of a logical system that represents the SAP S/4HANA system (Business System ID and IDoc Logical System ID) and a technical Hostname that belongs to the real communication partner on network layer, that is usually the middleware: SAP Cloud Integration for this course.
The communication system consists of 0...n system instances (~clients).
Creation of the Communication System
Technical vs. Logical Perspective
From the logical perspective each outer system of the figure (SAP S/4HANA and SAP Sales and Service Cloud) addresses messages directly to the other counterpart.
From the technical perspective the middleware is the first communication partner that receives messages and sends them further.
Hence in the communication system definition the business and logical system name (logical perspective) of the actual target system needs to be maintained, but the hostname (technical perspective) must be the middleware.