The entity system is the infrastructure that runs the game's simulation/gameplay code. (The gameplay itself is not part of this system - the system is not even specific to RTS games - but it is all built on top of this system, and influences the system's design.)

The design is based most closely on the one presented in GPG5 (Bjarne Rene: Component Based Object Management; Game Programming Gems 5, 2005, page 25). Some other useful references: ​Evolve Your Hierarchy, ​Game Object Structure: Inheritance vs. Aggregation, Scott Bilas's ​GDC 2002 presentation: A Data-Driven Game Object System, and a few of the things linked from those.