The Edit menu and the NN-XT context menu contains an item called "Sort Zones by Note". This option lets you automatically sort the selected zones in descending order according to their key ranges.
When you invoke this option, the selected zones will be sorted from top to bottom in the display starting with the one with the lowest range.
Note however, that the sorting is done strictly on a group basis. That is, only zones that belong to the same group can be sorted in relation to each other.
If two zones have the same key range, they are sorted by velocity range.