Since World War I, to conduct indirect artillery fire, three distinct components have evolved in this organization: the forward observer (FO), the Fire Direction Center (FDC) and the Firing Unit, sometimes referred to as the . On the battlefield, the field artillery team consists of some combinations of all of these elements. In other words, there may be multiple FOs calling in fire on multiple targets to multiple FDCs and any component may be in communication with some of the other elements depending on the situational requirements.