King's Regulations for the Army set out the various reasons (causes) for which a soldier could be discharged. In WW1, paragraph 392 of the 1912 edition of King's Regulations contained all the official causes of discharge, and these were set out in sub-paragraphs, numbered from (i) to (xxvii), omitting (xvii). In 1919 a new cause of discharge was introduced, numbered (xxviii).
The official causes of discharge, together with the subparagraph numbers expressed in both arabic and roman numerals, were as follows:
The cause and date of discharge can often be found in the soldier's Service Record. If he was discharged during the war and awarded a Silver War Badge, then this information can also often be found on his Medal Index Card and/or Silver War Badge List.
The reference to the relevant paragraph of King's Regulations can be written in many ways, for example:
On the National Archives website, if you look at the MIC for Private Albert Edward Blood of the 1/5th Bn Nottinghamshire and Derbyshire Regiment, you'll see that he enlisted on 8 Aug 1914, and was discharged on 7 Dec 1914. The cause of discharge was "p. 392 XVI KR Sick". Armed with the list above, we can see that paragraph 392 (xvi) is "No longer physically fit for war service". The "Sick" remark was often added to this cause to indicate that he was discharged due to sickness, rather than wounds.
King's Regulations had a set of notes for each of the above sub-paragraphs, and these gave special instructions to units regarding each reason for discharge. As war progressed, these notes were often amended, and the wording was changed. In some cases, certain sub-paragraphs were suspended for the duration of the war.
A few of the sub-paragraphs were divided into sub-sub-paragraphs, either before the war started, or during the war. Sub-paragraph (xvi) - the one most commonly encountered on MICs - is a prime example of one that changed several times. In August 1914, (xvi) was undivided. In March 1918 it was divided into (xvi)(a) and (xvi)(b), and at the same time (xvia) was created and this was also split into (xvia)(a) and (xvia)(b). In May 1918, (xvi)(a) was split into (xvi)(a)(i) and (xvi)(a)(ii).
If you wish to obtain more details about any of the above causes of discharge, click on the entry and you will be taken to a page which has extracts from King's Regulations 1912 (amended up to Aug 1914), and extracts from all the Army Orders that amended paragraph 392 of these regulations up to December 1919. Further amendments continued into 1920, when the whole table was revised, but most WW1 discharges take place before December 1919 and it was decided to stop at that point.