What you'd normally get out of a fluid source is pump (scalar) and pumpvel(vector) volumes, then you'd bring them in you DOP net using source volume initialized as pump, right? I guess what they wanted to do now is to simplify the naming. In 18 you can just add a velocity field that targets the vel field, and a pump field doing it's normal job of masking it. If you dive into, say, a smoke solver node, you should be able to find the node tree related to pump, and you should see that it usually refers to vel. Also, if you initialize Volume Source as a Pump, you should get a pump field and a v field targeting vel and masked by pump, since all pump is doing is adding velocity.