$UserBaseDirectory gives the base directory in which user-specific files to be loaded by Mathematica are conventionally placed.
$UserBaseDirectory 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 $UserBaseDirectory are:
These subdirectories are, if possible, created automatically the first time Mathematica is run by a given user.
Appropriate subdirectories are automatically included on $Path.
The subdirectories of $UserBaseDirectory are given in $Path before the corresponding ones of $BaseDirectory.