Perhaps it's created using
Associates a path with a drive letter.
SUBST [drive1: [drive2:]path]
SUBST drive1: /D
drive1: Specifies a virtual drive to which you want to assign a path.
[drive2:]path Specifies a physical drive and path you want to assign to
a virtual drive.
/D Deletes a substituted (virtual) drive.
Type SUBST with no parameters to display a list of current virtual drives.
Example, make the local drive G: refer to C:\Users\Graeme\Desktop:
C:\Users\graeme>subst g: c:\users\graeme\desktop
G:\: => C:\users\graeme\Desktop
Unlike mapped drives, virtual drives created using
SUBST do not persist between sessions, so it is typically used in a logon script if persistence is needed.