diff --git a/doc/user-manual/fpakc_UserManual.tex b/doc/user-manual/fpakc_UserManual.tex
index 7e7fdef..27c0a32 100644
--- a/doc/user-manual/fpakc_UserManual.tex
+++ b/doc/user-manual/fpakc_UserManual.tex
@@ -616,6 +616,11 @@ make
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
       \subsection{physicalSurfaces}
         Array of objects that links the named surfaces of the mesh to the models in \textbf{boundaries.particles} and \textbf{boundaries.EM}.
+        Each element of the array has the following items:
+        \begin{itemize}
+          \item \textbf{index}. Integer.
+            Index of the edge surface as defined in the mesh file.
+        \end{itemize}
 
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
       \subsection{inject}