Hi,
yes, this is a very good point and tricky to solved elegantly and expand to all kind of actions, it's difficult to know what is the input and output of some actions. sometimes there are several output possible as well, etc...
It could be that it's simply a case of placement and labeling. I do think that if we could flag with a attribute a field to be an "output", it would allow the action interface to indicate that visually, and prevent confusions.
Bye,
Jean