Matching loops (shown in example Maps/Looping/LoopSimple) are the easiest looping case. In this case, there is a loop in the output and a loop in the input (at the same level of looping), and you want an output element for each input element. Most of the time the matching looping is correctly calculated automatically when you map an input value to an output value. Here, the PurchaseOrder element in the input matches the Notice loop in the output.
To make a loop in the output match a loop in the input, use the SimpleLoop function in the Loop expression tab of the looping output map element. The Input Map Element argument takes the looping input map element. As with most of the looping functions, you can add filters and sorting as desired.