Editing Understanding the Function Hierarchy (section)

===Runs===

A run is the highest level of a Geant4 simulation. It is made up of one or more events. A user can define actions taken at the beginning and end of each event in the run action class file. Typically creating and saving the Ntuple used in the simulation occurs at the run level. The detector construction and generator action are also run level classes, even though they are only executed in the main. This means they are executed once per run, and they cannot change during a simulation. 

When running a simulation with the visualization package, the command to execute the simulation is run/beamOn <number>. The number is actually the number of events you wish to occur in this run. In effect, this command occurs at the run level as well.