The two internal modes are handled by the power management unit (PMU), whereas the external is handled by the user via the ENABLE pin.
The System Disabled mode provides a way to override the PMU by manipulating voltages presented to the ENABLE pin.
The PMU steers system-wide clock and power in order to provide the power modes - System ON and System OFF. Under the various modes, internal blocks are automatically powered by the PMU as required by the application.