I understand the confusion: it’s probably the ListOfPoints input port that needs some explaining. Because it could be that a duct needs more than 1 fitting placed along its length is why it expects a list of points. As you can see in your graph you are actually only supplying a single list. What you would need to do here (because you only have a single point per duct) is create a list of that single point.
Also for better performance you could try the Intersect nodes from the BIMorphNodes package. Check out this dyn for more info:
Mechanical fitting creation by wall intersection.dyn (22.0 KB)