This object identifies the physical entity for which the
sensor is taking measurements. For example, for a sensor
measuring the voltage output of a power-supply, this object
would be the entPhysicalIndex of that power-supply; for a sensor
measuring the temperature inside one chassis of a multi-chassis
system, this object would be the enPhysicalIndex of that
chassis.
This object has a value of zero when the physical entity
for which the sensor is taking measurements can not be
represented by any one row in the entPhysicalTable, or that
there is no such physical entity.