CCS = Coded Cab Signals, the current continuous coded track circuit system which dates to 1924 (and still used by Atk on the NEC, among other RRs).

ATS = Automatic Train Stop, the earliest version being the Vogt system of 1880. New installations prohibited by the FRA.

ATC = Automatic Train Control, an industry-generated term which sometimes confabulates the above two systems into varying mishmashes depending on the railroad company using them.

SC = Safety Control, usually applies to the deadman feature. When added to "CCS", as in "CCS-SC", it means "Speed Control", and the speed requirements of indications on the cab signal display are enforced by the onboard cab signal apparatus. This is a concept introduced to electric railways around 1910 by the Simmen System, a non-continuous system (there were others). Just to keep things clear, "CCS-SC" is occasionally referred to as "ATC".

The FRA no longer permits cab signals of a non-continuous nature, cab signals without speed control, nor new installations of ATS (because of its intermittent nature). Although they may use the same wayside and onboard signal apparatus and system designs, transit operations are subject to FTA (Federal Transit Administration) regulations, not the FRA's.

It rapidly becomes a case of the same acronyms being used by different railroads to mean different things.