The data dictionary for the registration reports outlines where each of the data points comes from in the registration reports. It can be very helpful to explain discrepancies in data and just to make sure you're referencing the correct report as a source of information. 

It can be found here - https://docs.google.com/spreadsheets/d/1icVWct65EhNnIXgnfSt_VslP8iwrZ6Rz8SztYzuNxw8/edit#gid=1997694548

Each sheet corresponds to a different report and is titled accordingly.