Related Rule
Practice Relating to Rule 108. Mercenaries
Section A. Definition of mercenaries
Belgium’s Law of War Manual (1983) states:
In order to be qualified as a mercenary, all the following conditions have to be fulfilled:
a) being specially recruited locally or abroad in order to fight in an armed conflict;
b) taking a direct part in the hostilities;
c) being motivated to take part in the hostilities essentially by the desire for private gain and, in fact, being promised by or on behalf of a Party to the conflict, material compensation substantially in excess of that promised or paid to combatants of similar ranks and functions in the armed forces of that Party;
d) being neither a national of a party to the conflict nor a resident of territory controlled by a Party to the conflict;
e) not being a member of the armed forces of a Party to the conflict; and
f) not having been sent by a State which is not a Party to the conflict on official duty as a member of its armed forces. 
Belgium, Droit Pénal et Disciplinaire Militaire et Droit de la Guerre, Deuxième Partie, Droit de la Guerre, Ecole Royale Militaire, par J. Maes, Chargé de cours, Avocat-général près la Cour Militaire, D/1983/1187/029, 1983, p. 23.