CPU core, AVX offset, FCLK, and Cache/Uncore multipliers allow you to overclock the CPU. The base clock of 100MHz is multiplied by each multiplier (ratio) and results in the final frequency. We have gone ahead and documented what each setting is called in each motherboard that might feature it.
MultiCore Enhancement is an easy way to overclock all cores to the maximum turbo frequency, which in this case is 5GHz. The issue is that multi-core enhancement adds too much voltage most of the time, and that overheats the CPU. However, you might get lucky and get a good CPU or motherboard that doesn't over volt the CPU. The stock all core turbo clock on the 9900K is 4.7GHz, enabling multi-core enhancement will increase that to 5GHz.
Start with 49x or 50x, and you can just type it in on most motherboards. On the Supermicro motherboard, you can set core six setting to what you want the rest of the cores to be at if you don't want to set each core individually. On ASUS, the board will auto-change to sync all cores if you set XMP to enable as it is enabling multi-core enhancement all the way.
While most games and other software don't use Intel's AVX instruction set, many stability testing programs and some software programs do. AVX uses parts of the CPU typically not used and as such will greatly increase power consumption, temperatures, and it could require more voltage to remain stable. The AVX Offset setting mas added a few generations ago to allow for different levels of overclocking depending on if AVX is detected in use. It allows you to set a number of multipliers for the CPU core to drop down to if AVX is engaged.
For example, if you set 50x for the Core ratio and -2 or 2 for the AVX offset, you will get a 5GHz overclock while playing games (no AVX) and 4.8GHz when running HandBrake (AVX). MSI lets you set a frequency for the CPU to shift to instead of a number of multipliers to go down when AVX is enabled.
The CPU cache, ring, and uncore are all the same thing, which mainly controls the frequency of the cache section of the CPU. Some vendors have turned cache up to 4.4+GHz by default, it depends on the CPU model. While you won't be able to match the CPU core multiplier most of the time, you should be able to stay 3-5 multipliers below the CPU core multiplier without requiring too much extra voltage. I would start with 4.4GHz cache ratio, overclock the CPU up to its maximum and then lower it a few to find a cache multiplier that doesn't require more VCore to stay stable. I should mention that VCore also provides power to the cache region and isn't a separate voltage rail like it was in the past.
VCore is your main voltage for stabilizing the CPU core and cache overclocks. Every vendor offers the ability to set the VCore to override mode, and it is the default on a few brands (like GIGABYTE), otherwise you have to choose. There are two other modes on most motherboards; adaptive and offset. If you run your CPU at maximum speed all the time, then override is what you want, but if you are going to let the CPU multiplier go up and down according to load, then you will want to set adaptive or offset mode. I will go over CPU multiplier dropping on the next page. The adaptive mode allows you to set a VCore you want as maximum and will drop down the VCore if the multiplier drops. It is better to use adaptive than offset, and it is easier to setup. Adaptive was introduced more recently, and before that we only had offset.
The offset mode allows you to set some millivolts to be added or subtracted to the CPU's default VCore for each multiplier. The new CPUs have different VCore levels for each multiplier (called VID), and every CPU has its individual default voltage levels. By default, Intel specification requires the VCore to drop by a certain amount when load and multiplier are increased. Load Line Calibration (LLC) reduces or reverses this default voltage drop, and it helps a lot when you are trying to stabilize the CPU.
If you want to set offset on a GIGABYTE motherboard, you need to type "normal" to unlock offset mode (offset is called DVID in their UEFI). Many motherboards will auto increase VCore beyond Intel specifications based on "auto-rules", so it's best to always set VCore, try 1.28-1.3v when at around 4.9GHz.
By default, motherboards should set FCLK to a multiplier of x8 (800MHz), but many vendors have set 1GHz by default. If your motherboard has it set at x8/800MHz, you should go and increase it to 1x or 1GHz, as it could help GPU performance slightly. FCLK typically does not produce instability when increased.