Originally Posted by
tismfu
I used to get SSSS all the time until I submitted a request for review and received a redress number. The security screening at international stations was usually easy. In comparison, if you’re connecting domestically, the screening is thorough and arduous (IME). They go through every last item of your carryons and you get a full pat down.
I think you're talking about TSA secondary versus CBP secondary. TSA secondary (SSSS) is very straightforward and happens before boarding, usually just a couple questions about where you've traveled recently and what your purpose of travel is. CBP secondary is usually after/during immigration and is the one where you'll have your carry on emptied and be patted down in a cold dark room.