Originally Posted by
bingocallerb22
But who adds the SSSS to the boarding pass? The airline? And under what criteria?
That's kept secret, since giving that information out would give the evil scary terrists information on how to defeat it!
Anyhow, it isn't published, nor will it be published. There are some things that seem to increase the probability of this (one-way tickets, purchased with cash, et al). But other than that, no one really knows, and those who do won't talk about it.