Ingredients can be classified based on our molecular weight. In this way, lighter ingredients will appear in a burst and quickly disappear.
Your middle or heart notes (couer in French) are really the centre of what your fragrance story is. They are the bridge between your top notes and your base notes.
But because notes and ingredients can be complex as well as relative, formulation by top, middle and base notes should be used as a guide to experiment and play with.