Internal PLL is used to stabilize high overclocks but at the risk of reducing the life of your CPU even more than just vcore & temps.
'Think of the CPU PLL voltage as a voltage that is provided to the CPU, but then “clipped” down to an approximate voltage. No matter what that input is whether 1.3v or 1.9v it is clipped (hypothetically let’s say 800mv after clipping (he didn’t say how much)) that way other devices can use the PLL voltage and clip to what they need. The CPU PLL Overvoltage allows for less clipping of that voltage. It can also reduce the lifespan of the CPU, but nothing noticeable.
So those of you who think that increasing your PLL voltage will help with that setting, it really doesn’t. But with SBe I have found that increased CPU PLL can help stabilize higher frequency overclocks. That wasn’t the case with SB.' Source is the guide
When disabled, the volts going to your CPU is clipped down by other components when there is extra. Enabled, the volts don't get clipped.
This is not PLL voltage in the Bios (if you have it).
VTT & VCCSA or IMC (Memory Controller or System Agent) are used when overclocking the memory. (leave on auto)
PCH should always stay on auto.
The guide I posted the link for is very helpful and one of the best, you just have to read up to post 7.