YARNRC is an optional environment variable that provides the name of a yarn
config file, overriding the default %HOME%/yarn/config.
It is useful in a couple cases. First, if you want to have two different
sets of folders/newsrc files but use the same config file for both, you
have two different values for HOME, but one of them can point YARNRC to
the config file from the other.
Second, if you are running yarn under two (or more) different OS's using
the same data, you can use the same HOME for both but point YARNRC to a
different config file pointing to the different .exe files (such as filter.exe)
necessary for the different OS. Someone posted an example of this recently.
I suppose you could also use it if you wanted to have a different color scheme
once in a blue moon.
-- BTW, A blue moon is actually the second full moon occurring in a single calendar month. This happens almost exactly 7 times in 19 years.