documentclass[11pt]{scrartcl}

usepackage{tikz}
usetikzlibrary{positioning,shapes,calc}
usepackage{pgf-umlsd}

begin{document}

begin{figure}


begin{tikzpicture}[every node/.style=draw,scale=1.25]

tikzset{part/.style={fill=gray!20,minimum width=1em,minimum height=1.4em,align=left}}
draw[fill=gray,minimum width=10em,minimum height=6em,align=left,anchor=south west]   (0,0) rectangle (8,4) coordinate[pos=.5](O);

node [above=.5em,part,xshift=1em] (part1) at (O){Part I};
node[on grid,part,below=2em of part1] (part2){Part II};
node[on grid,part,below=2em of part2] (part2){Part III};
end{tikzpicture}

end{figure}


end{document}