It means that the chokes have 60A per phase x 16 phases = 960A maximum
Actually the limit is probably 40A from the TI Mosfets = 640A
What does that mean for you? Asrock has a way overkill Mosfet.
That's good for temperature and lifespan of the Mosfets.
As far as the PWM, that means t hat they have a digital PWM controller that controls current to the Mosfets and the CPU.
It is looking though that the Asrock X370 has by far the most powerful VRM on their higher end boards. It's so powerful, you could probably run 5 Ryzen CPUs without any problems.
I actually thought it should be as good, if not better than Intel provided that the Ryzen controller was good.
This is the X370 Block Diagram:
Here is a Z270 Block Diagram
On X370 the CPU controls the M.2 board. On Z270, it has to go from the M.2 socket through the PCH to the CPU. The problem is that when it goes through, it shares the DMI 3.0 (the Z270 PCH to CPU link) with everything else connected to the PCH.