What do the following parameters mean in AMD overclocking settings: VDDC, VDCCI and MVDD?

VDDC (mV) - is the GPU core voltage 
VDDCI (mV) - is the I/O bus voltage (between memory and GPU core) and comes from the PCI-Express slot
MVDD (mV) - is the memory voltage

 

The default value for Navi GPUs is 1350 mV. The allowed range is 1250-1350 mV.

For Polaris cards the approximate value is 900 mV.