A legal concession is not always merely an optional convenience. In the second category, where survival genuinely depends on an available measure, making use of the concession can be required.
Example: A person facing death from hunger has only an otherwise prohibited food sufficient to avert the danger. The specific necessity permits the life-preserving amount; rejecting it simply because it is normally forbidden would be a misuse of the original prohibition.
Thus rukhsah is an overarching category with different practical consequences: in one instance an act is conditionally tolerated, while in another taking the permitted life-preserving measure is required.