In many applications, it is not easy to use a single model to describe the source.

In such cases, we can define a composite source, which can be viewed as a combination or composition of several sources, with only one source being active at any given time.

A composite source can be represented as a number of individual sources Si, each with its own model Mi and a switch that selects a source Si with probability Pi.

This is an exceptionally rich model and can be used to describe some very complicated processes.