There is a very good academy lesson that dives into this: https://academy.mendix.com/link/modules/22/lectures/122/4.1-Introduction-to-XPath
After a second look, what are you trying to do? A microflow can return a list. In your microflow retrieve a list of objects (with for instance an XPath) and the you could simply return that list. So if you retrieve a list of PlanningEntry called planningEntryList the return value would be $planningEntryList. You might want to follow some basic academy lessons.