Chapter 4 explains registration with the image of a bride leaving one household register and entering another. The point is not paperwork first. The point is transferred belonging.
In Book 9, restored people do not merely receive a new religious idea. They are portrayed as being relocated into the household and lineage of True Parents.
That makes registration relational before it is administrative. The doctrine sounds formal because it is trying to describe a belonging real enough to have names, inheritance, and home.