\documentclass{standalone}
\usepackage{tikz}
\usepackage{siltex}
\usetikzlibrary {arrows.meta,positioning,calc,math}
\tikzset{nodes={text height=.7em, text width=2.6cm, align=center,
draw=black!50, thick, font=\footnotesize, fill=white},
>={Stealth[round,sep]}, rounded corners, thick}

\pgfdeclarelayer{bg}
\pgfsetlayers{bg,main}

\begin{document}
\begin{tikzpicture}[on grid,y=1.1cm,x=2.9cm]
    \input{graphics/design_flow.tikz}
    \def\rectoffsetleft{-1cm,0cm};
    \begin{pgfonlayer}{bg}
        %\phasegroup(pd,id,Systems Engineering);
        \phasegroup(fs,fs,{})
    \end{pgfonlayer}
\end{tikzpicture}
\end{document}