When I start up a fresh executing Mathematica on my MacOS system and I open a Notebook file there is always one symbol defined in the Global context. This symbol is \$MySource and it points (via a directory symbolic link) to my default location for various notebook and package files.
I don't remember setting this up but I must have since it points to an iCloud folder that I myself created. I have searched all preferences and other locations such as account defined environment variables and I can't find this definition anywhere. I have searched the documentation for \$MySource and MySource but it leads to various content entries and none of those seem to contain the strings \$MySource or MySource.
If I change it to something different then when I shutdown Mathematica and start it up again it returns to the original definition to thru the directory link.
How do you set this value and does it have any special purpose other than providing a default file lookup location?