An Illumination Round is similar to Flares and Tracers, guiding paths and showing clearer visibility.
The M83A3 Illumination Round is seen briefly in the "Operation Guillotine Trailer" with the player loading it into a mortar and a marine saying "Get some illumination rounds in the air!" It acts in a similar manner to a flare, illuminating the ground below where it is fired.
In Singleplayer of the final product, the M83A3 Illumination Round is only used in Operation Guillotine, being placed in and fired out of an M224 Mortar by Campo. As the player proceeds through the level before reaching the apartment, more M83A3 rounds are seen fired. The rounds are distinctive by the red marking they leave behind, then exploding to illuminate the area.