Let $L$ = $\cup$ $Dk$ be a set of literals defined as the union of domains $Dk$ of simple datatypes (e.g., strings, numbers, dates, etc.). Let also $R$ and $P$ represent sets of labels for resources and properties respectively. A descriptive metadata specification is a structure ($G$, $R$ $\cup$ $L$ $\cup$ $P$, $F$), where: (1) $F$ : $(V$ $\cup$ $E)$ $â†’$ $(R$ $\cup$ $L$ $\cup$ $P$) can assign general labels $R$ $\cup$ $P$ and literals from $L$ to nodes of the graph structure; (2) for each directed edge $e$ = ($vi$ , $vj$) of $G$, $F$($vi$) $\in$ $R$ $\cup$ $L$, $F$($vj$ ) $\in$ $R$ $\cup$ $L$ and $F(e)$ $\in$ $P$; (3) $F(vk)$ $\in$ $L$ if and only if node vk has outdegree 0.