What are the conditions for the validity of wiping over the turban (Imamah)?

Primary text

For wiping over the turban to be permissible, it must cover the entirety of the head, with the exception of what custom permits leaving uncovered, such as the front part of the head and the ears, and similar parts on the sides of the head. This exception is granted because avoiding uncovering these areas is difficult by custom. This differs from a slight tear in the leather sock (Khuff), which is not excused. If a cap (Qinsuwa) is worn under the turban and some of it is visible, the apparent ruling is that wiping over both is permissible because they become like a single turban.