Is commonly called the "forward transfer function" because it is in the forward path.
Block H represent the gain of feedback component between the output and summer (error detector).
This transfer function is commonly called the "feedback transfer function" because it is in the feedback path.
The sum of the input signal.
Another important parameter of a servo system is the open-loop gain (transfer function),
which is defined as the gain from the error signal to the feedback signal.
Thus, it is the product of the forward path gain and feed path gain.