The VDS_DRIVE_LETTER_FLAG enumeration defines the set of valid flags for a drive letter.

 typedef enum _VDS_DRIVE_LETTER_FLAG
 {
   VDS_DLF_NON_PERSISTENT = 0x00000001
 } VDS_DRIVE_LETTER_FLAG;

VDS_DLF_NON_PERSISTENT:  If set, the drive letter no longer appears after the computer is restarted.