It’s based simply on appearance. So if something looks like water and has its properties, then it’s considered natural water. So tap water is fine. Now, chicken noodle soup or chocolate milk or extremely muddy water and probably even dyed water, oh, and urine, theologians consider those to be doubtful and can only be used in emergencies when you can’t get actual water. Another one is amniotic fluid. Can someone baptize a baby in the womb using the movement of amniotic fluid as matter? I believe that’s considered doubtful as well.