In some environments, it is required that users change a certain number of characters every time they create a new password. This is of course to prevent passwords from being easily-guessable, especially with knowledge of old passwords such as a departed employee might have for a shared service account.
I have separate questions open to address button-pushing side of this enforcement. However, I'm also curious as to how this enforcement works on the back end. If a cleartext password cannot be derived from a strong hashing algorithm, how does the system determine how many characters have been changed in new passwords?