If you connect your drive to the computer you get the drive letter of the first free letter. that can differ between the systems you use. Some people want/need a fixed drive letter for their portable drive. So the idea of some people was simple: Mount the portable drive to a virtual drive. Windows itself got all tools (normally) onboard. It is done by subst.exe.
Since version 2.0 its possible to configure a drive letter. There is a INI file named VirtualDriveSet.ini, which is located in the application main folder. Modify the variable Driveletter to your choice.
Update
Version 2.1
add un-subst
add lable for virtual drive
add paf like ini handling
Version 2.2
add configuration on first run
code cleanup
IF YOU USE A PAM MOD WITH EJECT FUNCTION, THE EJECT FUNCTION STOP WORKING AFTER STARTING VIRTUALDRIVESET!!!
Download VirtualDriveSet Portable 2.2
cu & hf
Dijaboro
ps: thx @ porwerjuice