Additionality is the thorn in the side of Development Finance Institutions (DFIs). It means: making an investment happen that would not have otherwise. Of course, everybody in development wants to make things happen that would not otherwise, and the possibility that aid substitutes for domestic efforts is a concern in other contexts. But additionality torments DFIs because of the constant suspicion that they crowd out private financiers by investing in products that would have been viable without public support.
DFIs are regularly called upon to provide rigorous evidence that their investments are additional. Rigorous quantitative evidence, in the eyes of academics, requires some credible method of estimating the counterfactual (what would have happened otherwise). And that requires something like a randomised control trial or a natural experiment or a valid instrumental variable. Yet, none of these is feasible in the world of DFIs. And without that, we are unable to distinguish correlation from causation.