What is Unmeasured output in MPC toolbox? It doesn't make sense

Hi, I’m confused with the term “Unmeasured output” on the MPC structure tab in MPC toolbox.

Generally speaking, in control theory, output is measurable so the term “Unmeasured output” doesn’t make sense.

Possibly, does the term stands for “Output that cannot be sensed because of not being able to attache a sensor or something” ?

Actually I’m getting stuck on the following problem ;

I wanna converge a second state to 0 in this LTI system using MPC controller

*There are four states

X =0.1366

Y =5.2133e-04

Z =0.0028

A=[0 0 1 0 ; 0 0 0 1 ; 0 0 0 0 ; 0 X/Z 0 0 ];

B=[0 ; 0 ; 1 ; -Y/Z];

C=[0 1 0 0];

D=[0];

inv=ss(A,B,C,D);

Input is single but Output depend on matrix C. Then I’m confused like

“Okay, well.. second state is measured output…the rest is …wait, what is unmeasured output??”