The latest hardware revision that's expected to be the mainstay for F9 flights going forward. Has better performance and addresses some reusability issues discovered in earlier blocks.
As far as I know, the center core was a block five one, but they didn't intend to refly it anyway, from what Elon said in the press conference. Maybe they intended to study it? If that's the case, there will probably be another "expandable" block five flight.
The plain is to lock down block 5's design so it can be man rated which in itself would make it more reliable. This will probably not happen until they fly them a few times though so that wouldn't mean the first block 5s would be more reliable.
