Like their distant cousins the Ogryns, Ratlings are abhumans, humans who followed a different evolutionary path, descended from ancient settlers cut off from larger human society. This is where the similarity to Ogryns ends, however. While "Ogryns are large, brutish creatures", Ratlings are incredibly short and round, and "remarkably clever".
Despite their "obvious unsuitability" for military life, Ratlings are nevertheless drafted wholesale into the Imperial Army. While they cannot fill many of the human roles in the Astra Militarum, Ratlings still proved incredibly useful. Their small size allows them to infiltrate enemy territory much better than the average human, and, combined with their incredible eyesight, makes them excellent snipers. In addition, Ratlings have a reputation amongst the Guard for their "incredible cooking", and often act as cook for the unit, in addition to their battlefield duties. Although, their love of food can often get them into trouble, as "many Ratlings are known to “acquire” extra supplies to supplement their diets, a practice their Imperial Guard commanders frown upon".
In the field, Ratlings operate independently, with a second Ratling who acts as a spotter and back-up marksman, or in small squads. They are deployed behind enemy lines with the barest of supplies, where they immediately go to ground, secreting themselves, living off the land, and waiting as patiently as spiders for their quarry. They delight in causing confusion and havoc among their enemies, killing officers and key personnel from extreme ranges, with near impunity. "Their proficiency and marksmanship may possibly be the Ratling’s one saving grace", and despite being the butt of numerous jokes over the millennia, many a Guardsman owes their life to the sharp eyes and steady hands of a Ratling sniper.