Eccoti il background:
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\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[latin1]{inputenc}
\usepackage[italian]{babel}
\usepackage{tikz}
\usetikzlibrary{shapes,arrows}
\usepackage{amsmath,bm,times}
\usetikzlibrary{shapes.gates.logic.US,trees,positioning}
\begin{document}
Innanzitutto non capisco a che serva questo codice, trovato in un esempio in rete:
\pgfdeclarelayer{background}
\pgfdeclarelayer{foreground}
\pgfsetlayers{background,main,foreground}
\centering
\begin{tikzpicture}
%% Draw system flow diagram
\begin{scope}[node distance=1.2cm,on grid,>=stealth',
elabor/.style={rectangle,rounded corners,draw,
text centered,fill=black!20!white},
results/.style={ellipse,draw,text centered,fill=black!20!white}]
\node [results] (a1) {risultato};
\node [elabor] (a) [above=of a1] {f(x)} edge [->] (a1);
\node [results] (b1) [above=of a] {y} edge [->] (a);
\node [elabor,text width=2.cm] (b) [above=of b1] {\emph{Step 2}.\\ -FindRoot-} edge [->] (b1);
\node [results] (c1) [above= of b] {ris.} edge [->] (b);
\node [elabor,text width=2.cm,fill] (c) [above=of c1] {\emph{Step 1}.\\ -FindRoot-} edge [->] (c1);
\node [node distance=1.8cm] (d1) [above= of c] {} edge [->] (c);
% Draw background
\begin{pgfonlayer}{background}
\path (d1) + (-3,0.2) node (e){};
\path (a1.south) + (3,-0.5) node (f) {};
\path [fill=yellow!20!white, rounded corners, draw=black!50!white,dashed] (e) rectangle (f);
\end{pgfonlayer}
\end{scope}
\end{tikzpicture}
\end{document}
`
Se i nodi avessero dei nomi più significativi sarebbe meno facile sbagliare la definzione dei vertici (e) ed (f) del rettangolo. I colori scegliteli tu.