If the data structures are the same then it is the order in which you add them to he input.
However, if some of your points have different paths and are then flattened then the order might be different from what you thought.
In the example below the order is meant to be start, middle, end. But because I have grafted the middle point it does not share the same path/data structure. And therefore the order is like that in the panel so when it is flattened the middle point comes last.
In order to make Path/data structures the same then there are some components that help
, Simplify being one see example below.