The M203 Smoke Grenade is a M203 round that delivers smoke at a greater distance than can be thrown. Once fired smoke will start developing from the round about 30 meters away from the initial firing point which can create a smoke trail if fired into the sky or with a significant arc (which can reveal the operators position).
While its use may seem little at first, it can prove very handy in case of an emergency. An arcing smoke grenade may be seen from several hundred metres away in daylight.