It just how the condition should arranged in that order within same block condition, with 'Trigger once' put at very bottom. With this setup the action will execute once, each time the condition met.
Say that you want to add value to a variable each time sprite1 (and it's instances) hit sprite2. So if the sprite1 overlap sprite2 at one time, the action will trigger. Another time they overlap again, the action will trigger again.
It will be different if you put 'Trigger once' at very top order. Try it by yourself