At a particular mode, there are 2 inflow arcs, a and b, and two or more outflow arcs, which is kept to 3 for this example, i.e., c, d and e
The first requirement is that only one of the two inflow arcs must be selected. Having said this, I am having difficulties modeling the following, as it differs from conventional flow conservation problems:
If arc a is selected, either one of the three outflow arcs must be selected, i.e, there is freedom to select any outflow arc.
On the other hand, should arc b be selected, only one of arcs d, e can be selected as outflow.