A nested loop (shown in example Maps/Looping/LoopSkipLevel) is the case where a loop in the output refers to a loop in the input that is nested inside of one or more ancestor loops in the input. In this case, the Notice loop in the output wants to loop as the Delivery loop from the input. The Delivery loop is nested inside of the PO loop in the input.
As with matching loops, nested loops are also automatically calculated as you map values from the input to the output.
To map a nested loop, use the SimpleLoop function in the Loop expression tab of the looping output map element. The Input Map Element argument takes the top-most looping input map element, then for each nested looping input map element, use the NestedContext function and then another SimpleLoop for the nested looping input map element. As with most of the looping functions, you can add filters and sorting as desired.