You will love the frequency domain after this, I promise

Looking back at the general controller architecture:

Then the complete transfer function collection is $$ \begin{bmatrix} y \\ \eta \\ v \\ u \\ e \end{bmatrix} = \frac{1}{1+PC}\begin{bmatrix} PCF & P & 1 \\PCF & P & -PC \\CF & 1 & -C \\CF & -PC & -C \\F & -P & -1 \end{bmatrix} \begin{bmatrix} r \\ d \\ n \end{bmatrix} \triangleq\begin{bmatrix} TF & PS & S \\TF & PS & -T \\CFS & S & -CS \\CFS & -T & -CS \\FS & -PS & -S \end{bmatrix} \begin{bmatrix} r \\ d \\ n \end{bmatrix} ,$$ where $S=\frac{1}{1+PC}=\frac{1}{1+L}$ is called the sensitivity function and $T=\frac{PC}{1+PC}=\frac{L}{1+L}=SL$ is called the complementary sensitivity function. They're called complements because $$ S + T = \frac{1}{1+PC} + \frac{PC}{1+PC} = 1.$$