For instance, when you define some parameter $\alpha$ and do not say anything, it is assumed that it is just a constant. However, if instead you write $\alpha_n$, is should be clear that $\alpha$ depends on $n$. To emphasize this one might even write $\alpha = \alpha(n)$. Similarly, to show that $2x^2$ does not depend on $y$, you could write $2x^2 \ne f(x, y)$, or $2x^2 \ne f(y)$. However, most people would not state that one quantity $q$ does not depend on some other quantity $p$. In the modern literature, authors bother writing something only if $q$ depends on $p$.