Mention that only two values of dB.order are in widespread use

chapters/annotations.tex

@@ -226,6 +226,11 @@ \subsubsection{Axis Properties}\label{axis-properties}
 It is recommended that value of \lstinline!order! is somehow indicated on the axis or its tick labels, for example by presenting a major tick label as $8\, \mathrm{dB}_{20}$.
 Major axis ticks are preferred at integer converted values, and minor ticks should be placed at evenly distributed converted values (similar to a linear axis scale).
 
+\begin{nonnormative}
+The two values of \lstinline!order! which are in widespread use are 10 and 20.
+10 is typically used when plotting signal power, and 20 is typically used when plotting signal amplitude.
+\end{nonnormative}
+
 \begin{example}
 A \emph{symmetric log} axis scale is sometimes used for axes spanning across several order of magnitudes of both positive and negative values.
 Details vary, but the mapping from value to linear position along axis is some variation of $y \mapsto \operatorname{sign}(y)\, \operatorname{log}(1 + \frac{\abs{y}}{10^{\alpha}})$.