A self-modifiable linked block is created when you add a self-modifiable masked subsystem block from the library to your model. Such linked blocks allow structural changes within the subsystem block.
You can observe that in this example, if you change the 'Time domain' on the mask dialog box, the block within the Subsystem is replaced accordingly. For example, if you select the 'Time domain' as 'Discrete', the continuous integrator block is replaced with a discrete integrator block and vise-versa.
This dynamic change in the structure of the self-modifiable linked block is controlled using the MATLAB code that is added in the Initialization pane of the Mask Editor.
The structural changes take place only if the option 'Allow library block to modify its contents' available on the Initialization pane is selected.