Clarify as suggested.

chapters/classes.tex

@@ -341,8 +341,10 @@ \subsection{Component Variability Prefixes discrete, parameter, constant}\label{
 \begin{itemize}
 \item
   A variable \lstinline!vc! declared with \lstinline!constant!\indexinline{constant} prefix remains constant during transient analysis, with a value that is unaffected by the initialization problem.
+  This is a called a constant, or constant variable.
 \item
   A variable \lstinline!vc! declared with the \lstinline!parameter!\indexinline{parameter} prefix remains constant during transient analysis, with a value determined by the initialization problem.
+  This is a called a parameter, or parameter variable.
 \item
   A \emph{discrete-time} variable \lstinline!vd! is a variable declared with the \lstinline!discrete!\indexinline{discrete} prefix, or a variable without any prefix that is not a continuous-time variable.
   It has a vanishing time derivative between events.
@@ -384,7 +386,11 @@ \subsection{Component Variability Prefixes discrete, parameter, constant}\label{
 
 \begin{nonnormative}
 Note that discrete-time \emph{expressions} include parameter expressions, whereas discrete-time \emph{variables} do not include parameter variables.
-This is consistent with restrictions on variability for expressions normally being upper limits.
+The reason can intuitively be explained as follows
+\begin{itemize}
+\item When discussing variables we also want to consider them as left-hand-side variables in assignments, and thus a lower variability would be a problem.
+\item When discussing expressions we only consider them as right-hand-side expressions in those assignment, and thus a lower variability can automatically be included; and additionally we have sub-expressions where lower variability is not an issue.
+\end{itemize}
 
 For \lstinline!Real! variables we can distinguish two subtly different categories: discrete-time and piecewise constant, where the discrete-time variables are a subset of all piecewise constant variables.
 The \lstinline!Real! variables declared with the prefix \lstinline!discrete! is a subset of the discrete-time \lstinline!Real! variables.