These records of service include those of engineers and paymasters.
Details of all naval officers' service had traditionally been recorded in books and registers. Around the outbreak of the First World War, a record-of-service card was introduced for officers in the Royal Naval Reserve. This concept, of a physically separate record for each officer, did not take long to become standard within the Admiralty, and personal files were soon appearing.
The Ministry of Defence subsequently decided that its most convenient arrangement for storing these individual records would be by date of birth, regardless of the arm of service. Records of officers from the Royal Navy, the Royal Naval Reserve, the Royal Naval Volunteer Reserve, and the Women's Royal Naval Service, therefore become intermingled. A further series (ADM 340) accommodates this mixture.
For officers who served in the First World War, records may exist in both forms - book/register and card/file - and may therefore be found in both ADM 240 and ADM 340. It is unlikely that any records in ADM 240 will relate to officers born too late for active service in the First World War.