The resource is no longer assigned to an incident and travel has started, but the resource has not yet arrived home.

The resource can be used to fill a request. Resources on preposition incidents qualify as available.

The resource is committed and at the assigned incident.

Extreme implies a level of fire behavior characteristics that ordinarily precludes methods of direct control action. One or more of the following is usually involved: high rate of spread, prolific crowning and/or spotting, presence of fire whirls, strong convection column. Predictability is difficult because such fires often exercise some degree of influence on their environment and behave erratically, sometimes dangerously. 

The burning of the foliage of a single tree or a small group of trees, from the bottom up.