To calculate fire safety distances between buildings, it is necessary to specify the location of the fire source in one of the buildings. The fire source can be any object on the scene (solid, wall, floor, platform) for which the Material property is defined. The exception is objects made of non-combustible materials. For details, see Materials Editor. Such objects cannot be the source of a fire.
To specify the location of the fire source in one of the buildings, select the desired object on the scene using the Pointer 
tool and in the object properties, check the On Fire checkbox.
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The selected object is set as the fire source for the current scenario.