The agent is an abstraction for whatever is being sensed by the sensor. So it is an abstraction for any physical phenomenon/property such as temperature, fire, pressure, intruder ...etc.
The sensors "sense" the agent, and upon sensing application packets are generated automatically by NetSim. This happens in n_NetSim_Zigbee_SensorEvent( ) in sensor.c. Users can modify this to set their own packet size/arrival times.
NOTE: From NetSim v11 onwards, the agent has been removed.