Simplified

chapters/classes.tex

@@ -346,7 +346,8 @@ \subsection{Component Variability Prefixes discrete, parameter, constant}\label{
   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 called a \firstuse{parameter}, or \firstuse{parameter variable}\index{parameter variable}\index{component variability!parameter}.
 \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.
+  A \emph{discrete-time} variable \lstinline!vd! is a variable that is discrete-valued (that is, not of \lstinline!Real! type) or assigned in a \lstinline!when!-clause.
+  The \lstinline!discrete!\indexinline{discrete} prefix may be used to clarify that a variable is discrete-time.
   It has a vanishing time derivative between events.
   Note that this is not the same as saying that \lstinline!der(vd)=0! almost everywhere, as the derivative is not even defined at the events.
   It is not allowed to apply \lstinline!der! to discrete-time variables.