RAM-speed vs CPU compatibility questions.

This doesn't seem to be covered in the RAM FAQ.

All CPUs have listed what type and speed of RAM is supported. For example Intel's Skylake Core-series is listed to support DDR4 at 1,866 or 2,133 MHz (and only those speeds).

So what happens if you stick such a CPU to a capable motherboard and pair it with RAM that's set to a substantially higher speed? Do you get trouble accessing the RAM, as should be expected? No, you get a performance increase!
What does Intel mean by publicly stating a limited compatibility when reality shows a much broader acceptance?

Then I have a side-note regarding XMP: The FAQ state that XMP stability isn't guarantied by the memory manufacturer. Yet the marketed speed for the memory I use (Crucial Ballistix Tactical, "2,666MHz") is achieved by activating XMP. With XMP off the RAM runs at a lower pace (2,400MHz).
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  1. To make things even more unclear:
    The RAM is supposed to be handled by circuitry in the CPU, the motherboard chipset should do nothing?
    * If so it's even more puzzling that faster RAM can be used at all...
    * Things get extra shady when you find that different motherboards featuring the same chipset (and CPU) provide different (theoretical) upper limits to the RAM speed supported...
  2. This is all about the technical aspects.
    * What clock controllers/where are involved?
    * What does it take to make it possible to set them at speeds above what's officially "supported"?
    * At which interface(s) can one expect to run into trouble while trying to maximize the RAM speed?

    - Do you need a "K" version CPU to "overclock" the RAM?
    - Can you do it with a "B" or "H" type chipset?
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