From the Geneva Convention Commentaries:

In short, all the particular cases we have just been considering confirm a general principle which is embodied in all four Geneva Conventions of 1949. Every person in enemy hands must have some status under international law: he is either a prisoner of war and, as such, covered by the Third Convention, a civilian covered by the Fourth Convention, or again, a member of the medical personnel of the armed forces who is covered by the First Convention. ‘ There is no ‘ intermediate status; nobody in enemy hands can be outside the law. We feel that that is a satisfactory solution — not only satisfying to the mind, but also, and above all, satisfactory from the humanitarian point of view.