\documentclass{standalone}
\usepackage{tikz}
\usepackage{siltex}
\usetikzlibrary {arrows.meta,positioning}
\tikzset{nodes={text height=.7em, text width=2.8cm, align=center,
draw=black!50, thick, font=\footnotesize, fill=white},
>={Stealth[round,sep]}, rounded corners, semithick}
% Compliant PDF
\usepackage[a-3u]{pdfx}

\begin{document}
\begin{tikzpicture}[on grid,y=1.2cm,x=3.2cm]
    \draw[fill=lightgray] (-1.7cm, 1.5cm) rectangle (1.7cm, -5.3cm);
    \draw[fill=lightgray] (-1.7cm,-5.5cm) rectangle (5cm, -9.2cm);
    \node (pd) {Problem Description};
    \node (sp)[below=1 of pd]  {Requirements};
    \node (id)[below=1 of sp]  {Initial Design};
    \node (fs)[below=1 of id]  {Feature Definition};
    \node (tp)[below=1 of fs]  {Test Protocol};
    \node (ss)[below=1 of tp]  {Feature Selection};
    \node (a1)[below=0.8 of ss,draw=none, fill=none] {};
    \node (rd)[below=0.8 of a1]  {Rapid Development};
    \node (va)[right=1 of a1]  {Variable-detail Approach};
    \node (pp)[above=0.8 of pd,draw=none,fill=none] {Preparation Phase};
    \node (dc)[below=1.6 of va,draw=none,fill=none] {Development Cycle};
    \path[->] (pd) edge (sp)
              (sp) edge (id)
              (id) edge (fs)
              (fs) edge (tp)
              (tp) edge (ss)
              (ss) edge (rd)
              (rd.east) edge[bend right] (va)
              (va) edge[bend right] (ss.east);
\end{tikzpicture}
\end{document}
