A registry is just a means of storing data. Large parts of it are inherently undocumented and subject to change. The registry was not designed so that it could be precisely defined, therefore, full documentation could not be clearly defined for it.
In certain situations (for example, "what values ββare valid in HKLM\SYSTEM\CurrentControlSet\Services\W32Time\Config\UpdateInterval ") you can find the vendor documentation (in this case, on MSDN).
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