A generic term for observation subjects who were gathered to become vessels for Fine's soul.
The biggest weakpoint of Fine's reincarnation system is that those who hold her seal carved into their genes must be exposed to an aufwachen waveform, it's the only point of the system that is left up to an overwhelming amount of simple chance.
Regardless of the fact that her vessels can inherit her paranormal powers and memories, if they are not in an environment where those abilities can revive, then idle time simply passes.
Thus, in order to overcome the weakness and prepare an "insurance," Fine began the project with the American government's F.I.S. If she lost the use of her current vessel in the middle of her plans, she wanted to assure she could revive without losing any time and restart her ambitions.
Thus the receptor children were her attempt to prepare her next vessel ahead of time. By searching through hospital records and personal information she isolated potential candidates, particularly those with thin ties to society, for example "those with no living relatives" (and in particular infants and children).
Above all, Japan had the highest amount of potential receptor children, and the reason for that is in about 722 BC a branch of a family in the Middle East that held the seal of Fine entered into ancient Japan.