About the difference between CAS and Digitization,

Digitization is the process of providing cable television signals in digital rather than analog mode. This requires television signals to be routed through a digital Set Top Box to the television.

The system used to control the distribution of digital television signals is called a Conditional Access System or CAS. This ensures that only the signals chosen and paid for by a subscriber are able to be received by that subscriber. So while digitization is between the mode of transmission of signals, CAS is about the channels that you wish to view.