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 virtual drive.
/D Deletes a substituted (virtual) drive.

Type SUBST with no parameters to display a list of current virtual drives.

e.g.
subst z: c:\temp

subst z: /d