Plural "Systems" in ECS architecture are the separate explicit and implicit modules (functions, objects, processing passes, etc.) that do something with the dat

Plural "Systems" in ECS architecture are the separate explicit and implicit modules (functions, objects, processing passes, etc.) that do something with the data represented by Entities and Components, and thus represent the third equal leg of the architecture, like in other three letter acronym architectures (MVC, BDI...); it isn't just an architecture organized as a system (singular) of Entities and Components, even if some kind of framework to manage them is necessary. Systems could be managed by a framework too, for example to process them automatically in a correct order.