Design Principles and Modularity
Coupling, cohesion, and the design principles that keep large systems changeable. You practice drawing module boundaries and use domain-driven design to make those boundaries match the business.
This domain covers the structural toolkit: cohesion and coupling as the forces that decide whether change stays cheap, the design principles that keep dependencies pointing the right way, and domain-driven design as the method for drawing boundaries the business will recognize. By the end you can decompose a system into modules and defend every boundary you drew.