What is the difference between the physical presence of impurity ('ayn al-najāsa) and its ruling (ḥukmuhā), and is what remains of stool under the fingernail after washing with water and soap considered the physical presence of impurity or its ruling?

If the impurity on the fingernail, after its substance has been removed by washing with non-pure water or by wiping, then touches something, that which it touches does not become impure. This is because the substance of the impurity has been removed, and only its legal ruling remains, which does not transfer. As for the impurity of feces under the fingernail, it is the substance of impurity and must be washed. It is not excused except when it is difficult to avoid due to hardship.