ID is "Induced Draft" and FD is "Forced Draft." In an induced draft system, the fan is at the exit end of the path of flow, and the system is under negative pressure - that is, the pressure in the flow area is below atmospheric, because the air is being drawn through the fan. In a forced draft system the fan is at the entry end of the path of flow. It operates at positive pressure because outside air is drawn into the fan and forced into the system.