But what does it mean?
Typically it means that fire can be seen from a window or the roof, but does that really mean that the strategic goal of ventilating the structure has been met?
Consider these words from Battalion Chief Kriss Garcia from Salt Lake City Fire Department, one of the leading proponents of positive pressure ventilation and positive pressure attack.
"Having fire come out of an opening is not ventilation. It is fire coming out of a window. Ventilation is the coordinated effort of the fire attack crew to remove products of combustion from the interior of the building while replacing it with a tenable environment (emphasis added) [...] Just because you have fire showing does not mean you have ventilation.You may have some but you have not taken control of the interior environment."
Consider this image of a residential structure fire.
Photo credit: FireTactics.net
Fire is "venting" (i.e., exhausting) from the D-side gable. But has the smoke been replaced with fresh air?
If yes, then why is smoke showing from the eves under pressure?
Remember, most "fire" fatalities are caused by smoke, so removal of smoke from a building should be a very high (and early) priority!
Not just an after-thought after the fire is out.
Our citizens aren't wearing air packs.