What does cross save mean if not cross play? You can use the same characters, but PC players and ps4 players do not see each other?
Sounds like cross-platform characters. So you can play the same characters on PC/PS4/Xbox (or not Xbox according to this). A lot of people have been asking for it as it's nice to play with folks on different platforms. A few dedicated folks have been running 6 characters across PS4 and Xbox for most of Destiny's life.
It makes sense if you're Sony to push for this, as it would allow Xbox players to jump ship throughout the game's life (incentive being strong exclusive content with each expansion and a larger playerbase, just like we have now) without feeling tied to their progress/gear/characters. One of the reasons I prefer to play on my Xbox over my PS4 is because I've put the time in there. In that light, it makes sense for MS to not want to allow that for the same reason. All with a grain of salt, of course. We'll see in 43 hours or so.