From what I've pieced together basically exo's are robots with a human consciousness/mind inside of them. They are either a male or female depending on what they used to be before their mind got transferred. Also humans created robots to perform repairs/chores/work. There are a few hints that they were possibly failed versions of exo's that had their conciouness erased or possibly just transferred to a working model of an exo. Then the failed exo design would be reprogrammed to work in the tower.