People distinguish various types of systems consisting of relatively autonomous functional parts.
The rules for identifying these systems are quite simple. For example, any system consisting of biological objects would be called an ecology, one made up of humans is a society, and a part of the latter that has to do with exchanging things is an economy. A contract economy looks much like a forest ecology, except that agents get locked into relationships by contracts, not niches, and some other manifestations of subsystems' intelligence. A general theory may also help to predict where new systems may have features qualitatively different from everything previously observed. Also digital systems often have lower replication costs of agents than execution costs, which makes them dramatically different from all systems more essentially embedded in their physical substrates.