The Repository Option Highwater Offset
lowers the Slot allocation below the total number of Slots currently allocated to this Repository.
For example if the total allocation of Slots to this Repository is 500, setting the Highwater Offset
to 100 will stop TMSS10SlotAllocation assigning any Volumes above Slot 400 (ie 500 total Slots - 100 Highwater Offset
Slots = 400 assignable Slots (1 to 400)).
Number of Slots to make un-assignable.