Disjunction only makes sense if different processes produce different outputs because they were created under different conditions. To make it clear that the output is a disjunction and not a conjunction, the direction of the nodes does not use a full arrow but a dashed line.

Exclusive disjunction is also assumed in this concept. Both rain, snow and hail can fall from clouds. And indeed sometimes it rains and snows at the same time. The latter we then call snow rain. If you were to use the inclusive OR with the input, you construct non-transitivity which makes the entire causal relationship ambiguous and unreliable. Not that this is really forbidden, but it is not analytical.