KATCOM, or in full Korean Attached Commonwealth Division, refers to significant numbers of South Korean soldiers who, during the Korean War, were attached to the 1st Commonwealth Division, similar to the KATUSA system in the US Army.
The system took effect from May 1952. Korean troops were included within Commonwealth units as replacements simply to make up numbers during periods of troop rotation. Soldiers assigned as KATCOMs were given 16 weeks' standard basic training at the ROK Replacement Training Center with further specialist training on British and Canadian weaponry. On average, each British battalion received 94 Korean soldiers, making a total of 1,000 Koreans across the whole Commonwealth division.
More broadly, KATCOM can also refer to Korean troops serving in other UN contingents, notably the Belgian and Dutch Contingents.