It is called fiato (breath) a certain precise movement of connection bewteen a pressure under the glottis and the voice, being that not a flow of air despite the name. It is a force, a pressure in the muscles, a lightness in the throat, a point of concentration, a posture given by a precise muscular movement.
There is only a way of being appoggiati and sostenuti, and it depends on a certain precise connection between diaphragm and larynx. You can hear it in a certain kind of overtones for the sustain and a related way for the appoggio, being that relation perfect when you can correctly execute a messa di voce (from pianissimo to forte or fortissimo and back to pianissimo). The way you perceive them and where you feel in the body depends on style, vocal range, individual body and body specifities, technical level (the higher level pass from a point of similarities of perceptions). All the places everybody can feel must work during the correct vocal gesture, so often in the training we have to work what you will never feel while singing, because it is the balance of all the places of diaphragm insertions with some wanted overacting that make the correct appoggio and sustain for style and voice.
On this subject one of the more common mistakes is to overwork the insertion you want to be heard in the sound, without noticing that it is the lack of some others that make its work not satisfing, the second being the fear for the overdoing of some specifics and the choice to make every voice simply and prudently just balanced without any specifics (losing in that way what of personal and truly specific a voice can produce in a deeper level).