The multi-ROI analysis module provides many characteristics of each object in a multi-ROI, among which we can see Surface Area X (or Y, or Z) and Surface Area X/2 (or Y/2, or Z/2).
The following chart illustrates what is the Surface Area on an Axis (X, Y, or Z):
If we compute only one side, it will be the half value (X/2, Y/2, Z/2).