  Our ability to control complex systems is a fundamental challenge of
contemporary science. Recently introduced tools to identify the driver nodes,
nodes through which we can achieve full control, predict the existence of
multiple control configurations, prompting us to classify each node in a
network based on their role in control. Accordingly a node is critical,
intermittent or redundant if it acts as a driver node in all, some or none of
the control configurations. Here we develop an analytical framework to identify
the category of each node, leading to the discovery of two distinct control
modes in complex systems: centralized vs distributed control. We predict the
control mode for an arbitrary network and show that one can alter it through
small structural perturbations. The uncovered bimodality has implications from
network security to organizational research and offers new insights into the
dynamics and control of complex systems.
