CPU VID Startup Value - sets what voltage is applied to your CPU during initial boot and for POST testing. Once you've past the BIOS POST tests, the voltage is changed to that determined by the next two values.
CPU VID Control - sets the base CPU core voltage
CPU VID Special Control - enables you to increase your CPU core voltage by percentages. Multiply "CPU VID Control" by this number to determine the core voltage applied.

People have noticed that in some cases they get a more stable overclock when the use a combination of CPU VID Control and CPU VID Special Control to set the CPU core voltage they want/need. Example: Instead of setting CPU VID Control to 1.5 and CPU VID Special Control to AUTO or 100%, they use 1.225 and 123%.

Other note: Non-default VDIMM values sometimes won't "stick" unless you have CPU VID Special Control set to something other than "Auto"

Peace