All of this is relevant in the scenario where you have a Parent task that is set to repeat

If a subtask under a repeating parent is  not set to repeat, that means it will  not be regenerated when the parent completes.  Once this subtask gets completed, that’s it.  When the parent is completed, a new parent will generate based on the repeat rules, but a non-repeating subtask will  not get carried over or regenerated under the next occurrence of the parent

If you want a subtask to be completed one time for every instance of a repeating parent task, then you want to set the child to “Repeat with Parent”. 

 To do this you will edit the subtask, then choose Repeat, then at the top, choose “Parent”, then after that, you will see the recurrence rule for the parent and you must choose that.  If you leave it at “none” then it will not get set. 

In rare cases you may want to create subtasks with different repeat patters from their parents.  This can be done.  You can set the parent task to repeat AND set a different repeat rule on a subtask.  When you complete the subtask, it will  immediately generate the next instance of that subtask (according to the rule you set for it) underneath that same parent. 

 Thus, that subtask will  always appear under the parent.  When you complete the parent, the current instance of that task is  carried over to the next occurrence of the parent task (this is notably different behavior than the norm…typically when you complete a parent task, all subtasks are marked as complete as well.  This is the one exception to that rule).

NOTE:  If you have an “End Repeat” rule set for the parent task, then  all subtasks will get completed and stay completed with that last parent task that is completed (since we are done repeating, there isn’t a new parent to “carry” a repeating subtask to)

I f you have a non-repeating parent, but a repeating subtask
The rules here are mostly the same as above, with the following exception:
  • The option to “Repeat with Parent” does not exist (the editor will not offer it to you)