Understanding Total Response
I've taken your diagram, and modified it just to show the negative feedback control system: -

Can you see that the only measurement made (red-box) is the output position.
For disturbance 1 I've used the example of power supply noise because it can add noise or variations to the output of the control system. However, to a large degree, that variation or noise is unknown &quot;to the system&quot; - all that the system knows is that it may have to act against those disturbing influences.
For disturbance 2 I've used the example of windage (the act of wind putting an extra burden on the plant). Again, those disturbing forces are not known &quot;to the system&quot;.