What are the impurities that are excused and their quantity, and does moisture transmit impurity?

The ruling on obsessive doubts (waswasah) concerning purification and other matters, and how to treat it, along with specifying the excusable amount of impurities: If wetness comes into contact with the impurity itself, then that wetness becomes impure, and everything it touches also becomes impure. However, if wetness comes into contact with a place that was impure, but the impurity itself has been removed from it, then that wetness does not become impure and does not transmit impurity, because what remains is the ruling of impurity, not the impurity itself, and the ruling of impurity is not transmitted.