What is the jurisprudential difference between expiation (kaffarah) and ransom (fidyah)?

Imam Az-Zarkashi mentioned that the expiatory payment (fidyah) differs from the atonement (kaffarah) in that the atonement is only obligatory for a sin that has already occurred, unlike the fidyah. The kaffarah is specific to a prior transgression of Islamic law, such as intentional sexual intercourse during a Ramadan day. As for the fidyah, it may become due without the occurrence of a sin, such as the inability to fast Ramadan due to old age or illness.