This has only recently been discussed in detail, you can find more info in this thread
https://forums.hardwarezone.com.sg/...l-overclocking-settings-6471527-new-post.html
In summary, there is an organization called JEDEC that certifies standards for RAM. JEDEC only certify certain most commonly used RAM speeds/timings. These settings are stored in the SPD of the RAM sticks, so when a RAM stick is plugged in, the mobo will read the SPD and select the highest settings that is supported by your CPU.
Now this is where XMP comes in. In the earlier days of enthusiast overclocking, users had to test different clock speeds and timings manually. At one point, Intel introduced the "eXtreme Memory Profile" (XMP), which is basically additional memory settings stored in the SPD. These settings are not JEDEC certified but Intel tested and verify. This allows the user to use stored overclock settings instead of tweaking it themselves. So RAM makers began to advertise this kind of RAM as being "XMP-ready". AMD later followed suit, they called it "D.O.C.P." because of copyright but it's the same thing - factory overclock profile stored in the RAM SPD.
Most RAM today run at max 2666MHz out of the box. To go higher you will need to either a) manual overclock, or b) activate the stored XMP/DOCP settings.
The speed is for each individual RAM stick. But if you have more than one stick, they will run at the lowest common speed. So if you have one 2666 non-XMP stick + one 3200 XMP stick, they will both run at 2666. For smoother operation, always buy RAM of same brand/model. That's what kits are useful for. Some kits even feature binned sticks i.e. they belong to the same test batch and so have highest compatibility with each other.
