An experiment consists of a TCL script, which binds the various elements used (the model system, input parameters, instruments) into an experimental run. An example experiment is ecolab.tcl. This is an executable script -- once you have made Eco Lab, you can run this script.
The script consists of several parts -- the first being a simulation loop which steps the model through the generate and mutate operators, then updates the various instruments (display, plot, connect_plot etc.) There is a running flag which controls whether the simulation is running or not. This is used by the run and stop buttons to control the execution of the simulation.
The other parts of the experimental script have been broken into separate TCL files -- model.tcl contains the input parameters to the model, and Xecolab.tcl has the TCL code relating to the X-windows interface.