Which are the criterias to recruit someone as a guard or a contestant?

By looking at guard 011’s story we see that she was living in precarious conditions similar to other contestants and she was also looking for her child. So this made me think that the guards have this element in common whit the contestants: a desperate need for money and poor living conditions. But why were they recruited as guards? I tought that military experience played a role because 011 was a soldier but player 120 was in the special forces and she was skilled in combat. So which are the criterias? Maybe that was explained in the series but I didn’t catch it.