Census UK 1939

1939 National Register - RG101

The following info is from http://1939register.info/.
The references displayed are in this format
Ref: RG101/1100C/019/36 Letter Code: CCVZ
where:
RG101 is the National Archives reference for the 1939 Register (this doesn't change)
1100C is the piece number
019 is the item number (it identifies a specific page but I couldn't see the number in the image)
36 is the line number on the page - again it's not shown, but you can count down

The Letter Code refers to the area - there's a guide to these codes in the TNA blog (http://blog.nationalarchives.gov.uk/blog/national-registration-day-29-september-1939/), and a list of codes at http://tidd.ly/cc7d398f
The following is from https://www.findmypast.co.uk/frequently-asked-questions/answer/what-information-is-listed-in-the--register.

The 1939 Register contains names, dates of birth, addresses and occupations of more than 40 million people in 1.2 million images.

The records list the following information: address, schedule number, sub number, surname, first name(s), OVSPI (for institutions only: Officer, Visitor, Servant, Patient, Inmate), sex, date of birth, marital status, occupation.

see also repository entry #95 for user guides to 1939 census