One of the great questions this. One of the thing I find the most misunderstood is that without sufficient support  it will be useless to work on copertura and appoggio. With sustain intending not just a force created by transversus abdominis and diaphragm but jalso the simple posture in tonic relaxed attitude, ready to sing. I generally find too short and need to be developed to work properly with appoggio and copertura. So I will say before sustain, when there is enough to have the correct larynx position we start working with copertura and appoggio with exercises involving also sustain (so to be sure not to lack of it totally).