ok, i got this idea from this other Freesource RTS (
www.imperialwinter.com).
essentially, it means that units can only fire upon enemies that are visible, whether in their sight or another's. this would allow for artillery spotters (guys who see the battlefield so that artillery can stay in the back, for commanders who can see farther than their men (thus giving them more to shoot at).
this means that the range of a unit could be larger than his sight, but he wont attack dudes unless they are in someone's LOS.