Which of the following options is NOT a use of illumination?

Illumination is primarily used in military operations to enhance visibility during low-light conditions, particularly at night. Its main purposes include lighting up areas where adversary activity is suspected, aiding in the adjustment of fires during nighttime operations, and sometimes employing it to harass enemy positions.

Marking friendly locations for Close Air Support (CAS) attacks does not align with the traditional uses of illumination. Illumination is not designed specifically to indicate friendly positions; rather, it serves to enhance visibility of the battlefield or to illuminate enemy positions for targeting. The use of other means, such as smoke or specific marking devices, is typically employed to mark friendly locations for CAS. This makes the choice of marking friendly locations the option that does not fit within the standard applications of illumination in military operations.