Some FAQ answers from FindMyPast:
Individuals’ records remain closed for 100 years from their date of birth (100 year rule). Records remain closed for people born less than 100 years ago until proof of death is verified. As more records become open as a result of the 100 year rule, more records will become available to search and view online at Findmypast. Of the 41 million records, approximately 13 million (around 32%) will be closed at publication. The 1939 Register will be updated regularly.
This record set only contains records of the civilian population for England and Wales, not the whole of the United Kingdom. Neither records for the Isle of Man nor the Channel Islands are included.
The 1939 Register was designed to capture the whereabouts of the civilian population on a specific date. Members of the armed services were not included in the register. The Register was not meant to record members of the armed forces and the records do not feature:
British Army barracks
Royal Navy stations
Royal Air Force stations
Members of the armed forces billeted in homes, including their own homes
However, the records do include members of the armed services on leave and civilians on military bases.