Keep in mind the current CCI-S is equivalent to the old CIC, which has always been an “international horse trial.” Basically the same HT run under FEI rules; the xc MAY be slightly more difficult than the national HT (the old “CNC”), but not always.
A CCI, now CCI-L, has always been a “Three Day Event”…meaning a longer, more challenging course requiring more fitness, experience, and qualifications. It’s “the big one” at the end of the season. Used to be R&T and steeplechase for a proper endurance day. But, yeah.
Current FEI suckitude is making less and less distinction between horse trial and Three Day. CICs (err, CCI-s) are just very expensive mandatory qualifier horse trials, not really anything to get excited about. The way I understand it, I believe the CIC concept was initiated to provide an international standard, so that less-popular countries (like Russia, small Euro nations, South American countries, etc) would be better prepared for a championship CCI. Their national competitions weren’t up to standard and they weren’t safe going to CCIs off their local HTs. Thus requiring CIC completions, guaranteeing experience at a corresponding level, made the competitors better prepared. This was/is less necessary in US, GBR, and other robust eventing nations where our local HTs are up to snuff. But, we all must suffer the quals for the lowest common denominator. And the FEI certainly likes taking our money by dipping into lower levels and requiring more FEI qualifiers. (Back in the long format days, FEI didn’t care about one stars, and left it to national federations to set qualifications. 4 clear HTs and you were good to go. Didn’t used to need a passport at 2star, either. FEI was primarily concerned with 3*/4* levels.)