What is the controlled variable in this particular case?
Assuming a state-space representation of your plant as follows:
$$
\begin{cases}
\dot{x}=Ax+Bu\\
y=Cx+Du\\
z\approx y+100
\end{cases}
$$
These are usual signal names you would find:

\$x\$: plant states
\$u\$: plant input / control signal
\$y\$: plant output / measurable output
\$z\$: controlled output / non-measurable output

In order to compose the feedback control loop, you are taking the plant output through an estimator system:
$$
\hat{z}=y+100
$$

\$\hat{z}\$: estimation of controlled output

I avoided using your \$y'\$ notation, since I think it's misleading for the explanation, but it should be equivalent as the \$z\$ or \$\hat{z}\$ signals in this notation.