The modelling world divides into those whose models are based on differential/difference equations (with or without disaggregation); and those who subscribe to an approach based on collections of objects (variously called object-oriented, individual-based or agent-based modelling).
Simile enables a population approach to be combined with a differential-difference equation approach. For example, a modeller might represent the vegetation in terms of compartments and flows, while the herbivores might be represented as individual animals, which are created, grow and die. In order to do this, a submodel is specified as being a population submodel (again, in its Properties dialogue box), and model elements can be added for specifying the initial number, and the rules for the creation of new individuals and the elimination of those already n the population. Visually, the submodel now appears with a shadow line for the top- and left-edges, and another for the bottom- and right-edges.