disturbances vs variations?in control system?
especially what is difference between \$\color{red}{\text{variations}}\$ and disturbances in a
control system?


For \$\color{red}{\text{variations}}\$, the quoted text in the question say &quot;plant variations&quot; and that is what I assume i.e. purely the plant (and not the load).
For disturbances I assume this to mean load variations rather than plant variations.

Does my interpretation make it clearer?
A control system tries to minimize these separate problems.
Plant and load (connected to output) distinctions: -

Picture from here.