Initial notes no drop vs clear [ci skip]

l3kernel/l3box.dtx

@@ -637,6 +637,32 @@
 %   \end{texnote}
 % \end{function}
 %
+% \section{Using boxes efficiently}
+%
+% The functions above for using box contents work in exactly the same
+% way as for any other \pkg{expl3} variable. However, for efficiency
+% reasons, it is also useful to have functions which \emph{drop} box
+% contents on use. This consideration \emph{only applies to local boxes}.
+% When such a box is dropped, the box becomes empty at the group level
+% \emph{where the box was originally set} rather than necessarily
+% \emph{at the current group level}. For example, with
+% \begin{verbatim}
+%   \hbox_set:Nn \l_tmpa_box { A }
+%   \group_begin:
+%     \hbox_set:Nn \l_tmpa_box { B }
+%      \group_begin:
+%      \box_use_drop:N \l_tmpa_box
+%     \group_end:
+%     \box_show:N \l_tmpa_box
+%   \group_end:
+%   \box_show:N \l_tmpa_box
+%   \end{verbatim}
+% the first use of \cs{box_show:N} will show an entirely cleared (void) box, and the
+% second will show the letter |A| in the box.
+%
+% These functions should be preferred when the content of the box is no
+% longer required after use.
+%
 % \section{Affine transformations}
 %
 % Affine transformations are changes which (informally) preserve straight