The serial number of the disk used to boot the system.
This is used for conflict resolution when multiple volumes want the same drive letter. Since LVM.EXE will not let this situation happen, the only way to get this situation is for the disk to have been altered by something other than LVM.EXE, or if a disk drive has been moved from one machine to another. If the drive has been moved, then it should have a different Boot_Disk_Serial_Number. Thus, the LVM Engine can tell which disk drive is the 'foreign' drive and therefore reject its claim for the drive letter in question.
If all of the claimants have the same Boot_Disk_Serial_Number, then the LVM Engine will assign drive letters on a first-come-first-served basis.