The correspondence in this series deals mainly with defence schemes,  plans and preparations. It includes reports made by defence committees, and remarks made on these by Lt. Colonel Bridges (Chief of
Intelligence), Captain Creswell (Director of Naval Services), Major-General Finn (Inspector-General ofthe Commonwealth Military Forces) and others.
The separate system of registration, annual single number with S prefix, for secret correspondence commenced in January 1905. From 5 January until 10 June secret papers received a dual registration, being allocated a number within the annual single number correspondence series and then given a separate annual single number with S
(Secret) prefix registration. For example, a report on a meeting of  the Council of Defence, received on the 10 May, was registered as 05/5018 and the secret registration, S05/85, was added in red ink. Papers were attached to files within the correspondence series and were known by the unclassified file number, with the exception of three files (S05/66, S05/85 and S05/101) relating to the Council of Defence, which were knownby their secret registration.
From 12 June 1905 the practice of dual registration ceased (except  when the decision to classify a paper as secret was made after its initial registration) and secret files were known only by their secret registration -starting with S05/109.
The series was a small one, and sometimes files were identified by the earliest correspondence registered and sometimes by the latest. For example, the file known as S06/54 contains remarks by the Chief of  Intelligence on a report by the Committee of Imperial Defence in May 1906. The remarks were registered as S06/54, from which the file number was derived. A subsequent piece of correspondence noting that the remarks had been sent to the Governor-General was registered as S 06/66 and attached to the file S06/54, i.e. the file was known by its earliest registration. On the other hand the file known as S06/58 deals with remarks on the same report from a variety of sources registered as S06/55, S06/56 and S06/58, i.e.the file was known by the latest registration.
The secret registration was marked on the papers in three ways. Until October 1905 registrations were marked in the purple oval commonwealth defence stamp used for the unclassified correspondence series. From October an oval red secret defence stamp was used. From April 1907 papers were marked with a purple oval secret defence stamp.
Registrations, with an S prefix, in each instance were placed inside the stamp in red ink.