If I am over water there may be a upper number of 100 and a lower number of 004. IF there is a 400´hill, there is for certain another of 300´ and a lower bank of 100´ etc. We often see values higher than the beaches that we are flying past.
The lowest terrain...strange... I think this number is more for us to evaluate the type of terrain ahead. If I see a 150/70 I know there are some highrise mountains ahead but if I see 100/90, there may be more of a plateau coming up.
I can´t see why we need to know the lowest terrain except for the above speculation.