The best material to work on is something that is visibly already taking place in the person's everyday life, that needs improvement. It is certainly an active subject if she is living it. So, if the client comes in and states what she currently has problems with in life, then we can be sure that it is an active subject ready to address and resolve.
Life tends to bring up stuff that people ought to be working on. For that matter we could say that that is the whole purpose of life. It presents situations that represent something the person has to learn. Generally we can be sure that if life has called a subject to the person's attention then it is primed and ready to learn from.
A subject that is active in life would be pretty much anything that the client would find important enough to bring up as a concern when you start the session. It could be a problem, an upset, a difficulty, a lacking ability, a desired outcome. That the client brings it up means that it is active.
If the client doesn't bring anything up then it will be up to the facilitator to activate something.
The facilitator can only activate areas that are potentially loaded for the client. The client must at least have a potential that she currently isn't aware of. The facilitator will call attention to such an area and thereby it will become activated. The facilitator might guess at an area that is a candidate for activation, or she might simply pick a general area from a list of likely subjects.
Never work on a subject that isn't activated. The client's interest will be the measure of that. If she is interested in improving the area then it is active. If she is not interested in it, it isn't active.