mar/graphics/functional_relation.tex

%&tex
\documentclass{standalone}
\usepackage{tikz}
\usepackage{siltex}
\usetikzlibrary {arrows.meta,positioning,calc,math}
\tikzset{nodes={text height=.7em, text width=2.5cm, align=center,
draw=black!50, thick, font=\footnotesize, fill=white},
>={Stealth[round,sep]}, rounded corners, semithick}
\pgfdeclarelayer{bg}
\pgfsetlayers{bg,main}

\begin{document}
\begin{tikzpicture}[on grid,y=2.2cm,x=5.7cm]
    \node (re) {Requirement};
    \node (fu)[right=1 of re]  {Function};
    \node (co)[above=1 of fu]  {Component};
    \begin{scope}[nodes={draw=none, fill=none, text width={},text height={}}]
    \node(c)[left=0.5 of co.north] {}; 
    \node(d)[above=2mm of c, anchor=south] {Elements of a Feature};
    \end{scope}
    \begin{scope}[nodes={draw=none, auto, fill=none, midway,text width={},text height={}}]
        \path[->] (re) edge node[below] {Specifies} (fu)
                  (co) edge node {Performs} (fu)
                  (re) edge node {Specifies} (co);
    \end{scope}
    \begin{pgfonlayer}{bg}
        \coordinate(a) at ($(re.south west) + (-2mm,-2mm)$);
        \coordinate(b) at ($(co.north east) + (2mm,2mm)$);
        \draw[fill=lightgray] (a) rectangle (b);
    \end{pgfonlayer}
\end{tikzpicture}
\end{document}