I'm dealing with multiflows and I found in "Combinatorial Optimization - Part C" by Schrijver in Chapter 70 a good source. The definition of the multiflow problem involves so-called supply- and demand-digraphs:
"Given two directed graphs, a supply digraph $D = (V,A)$ and a demand digraph $H = (T,R)$ with $T \subseteq V$. a multiflow is a function $f$ on $R$ where $f_r$ is an $s-t$ flow on $D$ for each $r = (s,t) \in R$. ..."
The problem I have is that I don't understand their definitions. Most of the books dealing with multiflows just regard one graph. For example, regard the digraph below, what are $D$ and $H$? What are the benefits of Schrijver's definition?
Thanks in advance!