A service number is an identification code used to identify a person within a large group. Service numbers are most often associated with the military; however, they also may be used in civilian organizations. National identification numbers may be seen as types of service numbers.
The term "serial number" is often seen as a synonym of service number; however, a serial number more accurately describes manufacture and product codes, rather than personnel identification. In the Canadian military, a "serial number" referred to a unique number assigned each unit that mobilized for the Second World War.
Australia
In the First Australian Imperial Force soldiers were allotted numbers known as regimental numbers. These were allotted to NCOs and other ranks but not to officers or nurses, who had no numbers. Regimental numbers were rarely unique. Each battalion or corps had its own sequence, usually starting at 1, although some units were formed in the field and this did not occur. The result was that several dozen soldiers had the prestigious number 1, which was usually given to the Regimental Sergeant Major or the Regimental Quartermaster Sergeant. When soldiers were transferred from one unit to another, they often kept their number if it was not already held by someone else. Otherwise, they might be allotted a new number or the letter A or B might be added to make the number unique with their unit again. Re-enlisted soldiers often used the additional letter R. In 1917, the AIF switched to a scheme whereby reinforcements were drawn from the common pool instead of being supplied on a per-unit or corps basis. These were known as "general reinforcements" and they were allotted unique numbers in the range of 50000–80000. Despite the limitations of the scheme, in researching a soldier, it is handy to know the regimental number.
In 1921, the assignment of identifying numbers based on regiments was abolished in the Australian Army and replaced with an Army-wide system. This meant that soldiers in all branches of the Army received a unique number, and if they re-enlisted they kept their previously allocated number.
The problems inherent in the First World War scheme were acknowledged and all members of the Second Australian Imperial Force were allocated a unique service number known as an Army number. The first letter represented the state of enlistment: N: New South Wales; V: Victoria; Q: Queensland; S: South Australia; W: Western Australia; T: Tasmania; D: Northern Territory. The serial numbers of female soldiers followed this with an F. AIF serial number then had an X. A low number indicated an early enlistment. General Sir Thomas Blamey was VX1. Soldiers transferring from the Militia often kept their old number with 100,000 added, while PMF officers had 200,000 added.
Following the Second World War, the system employed by the Australian Army was quite complex, as the Second AIF was disbanded and an Interim Army was established. A dual system existed until July 1947 as existing personnel kept their Second AIF numbers until that point, while new enlistments received a service number starting from X500000, while maintaining the state-based prefix of the old Second AIF system. For the second half of 1947, numbers were allocated starting from X700000, again prefixed with the state of recruitment. However, from September 1947, another system based on military districts was introduced for members of the Australian Regular Army, while personnel who enlisted specifically for service during the Korean War with K-Force received numbers beginning from 400,000, although those who volunteered for service with the Regular Army Supplement received a new number beginning with 900,000 and others who transferred from the Interim Army to K-Force received a new number beginning with 905,000, beginning with a number from 1 to 8 representing each state and territory (beginning with Queensland), including Papua New Guinea. This system (the 900,000 series) remained in use until 2002, when it was replaced with an integrated system based on the Personnel Management Key Solution (PMKeyS) system, which moved the Australian Defence Force away from service numbers to employee numbers.
Canada
Canada began using "Regimental Numbers" during the First World War.
During the Second World War, units were allocated blocks of Regimental Numbers to issue out, usually in the 5 or 6 digit range, though extremely low numbers were also possible due to the blocks.
X12345: the X was an alphabetic character denoting the Military District the soldier was recruited in (A represented MD1, B MD2, etc.) Up until 1945, officers never received numbers and were identified by name and rank only.