'Working fire" is describing the amount of fire on a structure. In book terms, a working fire is defined as a fire that involves 75% or greater to a structure (residential or commercial), substance (combustible fuels, chemicals, brush, etc.) transportation vehicle, (car, truck, aircraft, train, etc.) or any large container, (dumpster). But to make this definition more user friendly, let's just say that any fire that's coming out of every window or through the roof of a residential or commercial structure is classified as a "working fire". Now from a communications standpoint, hearing "Working fire" on a fire company level means that it's time to work agressively for about the first 20 minutes. Whether it be laying a supply line to the attack pumper, assist the attack crew or do truck work.