In order to make a child record added to a parent record in the custom record setup ALLOW CHILD RECORD EDITING should be checked.
If the ALLOW UI ACCESS is checked we can create standalone ones for child record. But if its not checked we can only access the child record via its parent record UI, so to restrict their creation through in line editing and we can strictly link parent and child records in this way.
Thus it looks like the item sublist, because as shown below such a button in the sublist will vanish.
