Regestation is the process of fully encasing a subject in a VR Womb for a long period in order to drastically and permanently change their brain chemistry.
In the case of Project Orange Powder, it was simply used to designate promising subjects for genetic drift. Clones were discarded after their testing data was complete.
But some people undergo this process electively, in order to learn a new skill, beat an addiction, or any other hard process that requires mental plasticity and effort.