A pvp session will also end if the host enters a boss room, or if the invading guest player uses a Silencing Blank to return to their own world. You'll have to re-summon for each area which can take some time.
I love the way the bells work in the rest of Bloodborne, and I like how mysterious and holistic it all is. You can also interact with other Bloodborne players in a more traditional way, at least in theory.
The other major change, in my mind, is that players are now able to hold Blood Vials and Quicksilver Bullets in storage. Adjusted the effect of Beast Blood Pellet item. Vilebloods will not summon Executioners and vice versa - at least the chance of being summoned will be very low.
A Level 4 can summon a Level this way. Getting the game to match you with one of your PS4 friends requires some weird mixture of luck, careful communication, and digital dark arts.
Inside System, go down to Network. Blood Vials and Bullets don't refill when you return to your world.
To whoever might be working on those patches: Keep the story areas the same. Rather, to join other players, you must ring one of a series of bells, which are actual in-game objects that your character carries around.
It's useful if you're just a couple levels outside each others range. Messengers play a big part in online gameplay.