The Persistent World Save System is a saved game editing (SGE) system that allows persistent worlds to merge a saved game with its corresponding module. This allows for persistence of local variables, PC and creature location and status, journal entries, and basically everything else necessary to make a persistent world truly persistent. While there are several alternatives already out there, they all rely on a fairly complicated system of log editing or item storage on players.
Previously the problem with editing saved games was that it is a fairly difficult process to go through, and for a large PW it would take an insane amount of time to copy all the variable tables for every single creature. Since PW_SvSys does it all for you, this is no longer a problem.