Non-normatively describe common meaning of 'structural parameter'

chapters/classes.tex

@@ -441,6 +441,13 @@ \subsection{Component Variability Prefixes discrete, parameter, constant}\label{
 \end{lstlisting}
 \end{example}
 
+\begin{nonnormative}
+Related to evaluable parameters, the term \firstuse{structural parameter} is also used in the Modelica community.
+This term has no meaning defined by the specification, and the meaning may vary from one context to another.
+One common meaning, however, is that in the context of a given tool, a parameter is called \emph{structural} if the tool has decided to evaluate it because it controls some variation of the equation structure that the tool is unable to leave undecided during translation.
+With this interpretation of \emph{structural parameter}, it follows that such a structural parameter must also be an evaluable parameter, while there are typically many evaluable parameters that are not structural.
+\end{nonnormative}
+
 If a \lstinline!Real! variable is declared with the prefix \lstinline!discrete!\indexinline{discrete} it must in a simulation model be assigned in a \lstinline!when!-clause, either by an assignment or an equation.
 The variable assigned in a \lstinline!when!-clause shall not be defined in a sub-component of \lstinline!model! or \lstinline!block! specialized class.
 (This is to keep the property of balanced models.)