I'm looking to create separate workflows for a subtask and parent task within a task list. What I'm trying to accomplish is that only the parent tasks will create a document set for that task within another library, and the sub tasks will only send a email notification, I have so far found no way of setting a condition as for parent tasks or sub tasks as there is no column I can find that determines a difference.
Nintex does not recognize the List items as "parent" & "sub" tasks. They are just individual list items. I have gotten around this by creating a new Yes/No field called "Parent". When I create a "parent" task, I check the box and leave it unchecked by default for sub-tasks. This will let you set a criteria/IF action in Nintex WF for creating a document set OR sending a notification.
Also, I use this to create SP Lists views that only show my parent tasks. (i.e. create a Calendar View that shows all "projects" that have Parent field checked.)
I hope this helps resolve your issue.