Sorry folks, to interrupt:
you're wondering about the 0.2ppm SI / absolute / Klitzing Ohm...
It's quite easy, and still actual, as somebody else already mentioned:
Currently, the SI Ohm is uncertain to about 0.2ppm only!
Same goes for the SI Volt!
That's due to the current definitions (from 1990) and/or mise en pratique, derived from other SI base units, like the kg, frequency, length, and several "unprecise" constants of nature.
Best mise en pratique of the Ohm (in terms of SI units) is the Thompson-Lampard capacitor ( calculable capacitor).
The Ohm is NOT defined or derived from the Hall / von Klitzing standard; at the moment latter one is ONLY the best way to have a repeatable, site independent standard.
The von Klitzing constant for the Hall effect resistance (combination of e and h) currently is an approximation only, also 0.2ppm uncertain relative to the SI.
Therefore,the absolute uncertainty as per today can not be any better than these 0.2ppm, nowhere!
Therefore, the NIST declaration is very reputable..
In 2018, probably a new SI will be adopted:
The kg will be defined by the silicon sphere and/or by the Watt balance.
Several constants of nature will be defined with zero uncertainty, especially h and e.
As a consequence, the Ohm and the Volt, defined AND mise en pratique by the Klitzing and the Josephson quantum effects/standards, will also have zero uncertainty!
That means, that both effects are already proven to be 1e-17.. 1e-19 stable or reproducible on cryogenic level. (That is: SQUIDs for null detection, superconductivity, no thermal voltages)
That will be as stable as the second / frequency quantum standards (Hg or Al optical clocks).
The problem to connect these cryogenic standards to the real world, will still be persistent.
The difference between both worlds are thermal voltages of the order of several nV, which limit the achievable real world uncertainty to about 1e-8.. 1e-9, but that will be 1 or 2 orders of magnitude better than before.
What's lacking also, are working / secondary standards for Ohm and Volt, being as stable as << 1e-7.
Therefore, even when the new SI 2018 will be gone into effect, there will be no big improvement in practice, as we will certainly still discuss the stability / T.C., etc. of PWW and BMF resistors...
Frank