In humanitarian law, what does the principle of proportionality address?

In international humanitarian law, proportionality prohibits an attack whose expected incidental harm to civilians (collateral damage) would be excessive in relation to the anticipated concrete and direct military advantage (Additional Protocol I, Article 51(5)(b)). Whether force may be used at all, or whether a conflict is lawful, is a question of military necessity or of the law on the use of force (jus ad bellum), not of proportionality in attack.