If any threat from the treacherous lower world Pulse is found on Cocoon, then the Sanctum enacts the Purge to rid Cocoon of this terror. Sadly, they do not discriminate when it comes to those deemed contaminated and those just in the general area where a threat is found will be condemned to the Purge.
Loading everyone on to special Purge trains, PSICOM guard those contaminated until the train reaches its destination at The Hanging Edge. It is said that those for the Purge will be exiled to Pulse below but evidence comes to light that at journey's end, they will in fact be exterminated. This is covered up by The Sanctum because of the potential political fallout from the knowledge that the government is cruelly murdering its own people. This discovery is precisely what sparks the events of Final Fantasy XIII and is where the journey begins.