The agent is an abstraction for what ever 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.