In one article I found the following notation without explanation, only statement that it is "the usual one in graph theory".
Let $e$ be a edge in directed graph and suppose that $v$ is vertex in this graph. Then authors use notation $v\in t(e)$ and (in another place) $v\in s(e)$. From the context I know that it should mean "$v$ is the begining of $e$" and "$v$ is the end of $e$", but I don't know which one has first meaning and which the second one. I tried to found this notation in many places but without success. I suppose $s$ and $t$ are abbreviations, but I don't have any idea from which words it follows. (is $s(e)$ a starting point ?)
Most books I've seen on flow/algorithms use s to be the starting point and t to be the target of the given edge. It is strange, however, that this was expected to be trivially understood, as it's not the notation across the board.