Q: What is the difference between this module and the Session API module?

A: Session API creates alternative session ids that remain valid after authenticated users log out and become anonymous. Session API does not implement any session data storage mechanisms, whereas that's what Session Cache API is all about. Session Cache API sessions also continue to live after logout, so you normally don't need Session API when you already use Session Cache API. See the README for more info on sessions, Session API and Session Cache API.