It refers to the amount of the perimeter that the fire (probably) can't cross. https://www.npr.org/2017/12/21/572305828/what-does-containing-a-fire-really-mean

It refers to the amount of the perimeter that the fire (probably) can't cross. https://www.npr.org/2017/12/21/572305828/what-does-containing-a-fire-really-mean https://www.npr.org/2017/12/21/572305828/what-does-containin... > That means that [x] percent of the fire is surrounded by containment lines — any physical barrier that stops the fire from passing a certain point. Those lines can be trenches, natural barriers like rivers, or even already-burned patches of land. Most often, the containment line is a shallow, 10- to 12-feet wide trench firefighters dig into the dirt.