Question: Please explain me the rules of used water, when a water is called used?
Answer: Water becomes used by removing hadath, or by being used for worship. When it has been used for worship (with the intention of worship), such as wuduu', ghusl or washing ones hands before eating, then the water is used even in the second or third wash, due to the intention. Also, water becomes used by removing hadath, even if it was without intention, such as if someone took wudu' and ghusl without intention. If one took wuduu' without intention, and didn't already have wuduu', then the water of the first wash would become used, not the second wash, because the hadath was removed by the first wash and there was no intention of worship in the second wash.
... and Allah knows best.
