We use "Ya sas" , when we talk to many people or when we want to be polite with a person (no matter if the person is a woman or a man). In other words it is the plural of "ya sou" and it's the polite form.
We use "Ya sou", when we talk to a child or a person that we already know good. Usually we say ya sou when the other person is younger or has the same age with us...