$BaseDirectory gives the base directory in which system-wide files to be loaded by Mathematica are conventionally placed.
$BaseDirectory returns the full name of the directory as a string.
Typical values are:
The value of $UserBaseDirectory can be specified by setting the MATHEMATICA_USERBASE operating system environment variable when the Mathematica kernel is launched. It cannot be reset from inside the kernel.
Typical subdirectories of $BaseDirectory are:
These subdirectories are, if possible, created automatically the first time Mathematica is run.
Appropriate subdirectories are automatically included on $Path.
The subdirectories of $BaseDirectory are given in $Path after the corresponding ones of $UserBaseDirectory.