Fractal branching models - frequently called Lindemeyer or L-system models - work by repeated application of the same branching rules. The Simile implementation uses a population submodel for the branch segments (intenodes), starting off with a single member (the base segment). An association submodel is used to define the relationship between one segment and its two daughters.
The behaviour of the model is visualised using a specially-developed display tool, showing the 3D structure of the simulated tree from the side and from the top.