The series consists of the authorized list of subject headings for the new 'secret' series implemented in Navy Office in 1923 after the abandonnment of the annual single number series system. It is titled :                'Navy Secret Registry.
                Index to Series - 1923-1950.
                Alphabetical.'
 
The primary subjects range from ACTS to WORKS, each subject being allocated a number starting from 1802 and finishing (originally) at 2017. The numbers were allocated in alphabetical order, but not all numbers were used, eg.1803 and 1805 are not represented. This was probably to allow for the addition of new subjects at some later date.  
New subjects have been added over the life of the series, especially during World War II, and the gaps left were apparently not always sufficient for the purpose. For example, the subject 'CASUALTIES' was allocated number 1825 between subjects 'BASES' (1824) and 'BOARDS AND COMMITTEES' (1828), and the second last subject in the alphabetical run, 'WIRELESS', has the last number (2037).
 
The primary subjects are often sub-divided into sub-categories and allocated secondary numbers, eg.1880 'FUEL' has secondary subjects: /2 GENERAL
/3 COAL
/4 OIL
/5 RN
The primary subject 'SHIPPING' has 32 secondary categories, while most have only one or two.