This is necessary to be able to rotate assemblies. just place the hotspot somewhere inside the voxelspace and it will be possible to rotate a voxel space and place it at the same position without knowing the size of the piece The hotspot is also transformed, when the piece voxel space is transformed The hotspot is in the centre of the given voxel for most of he defined voxel spaces. This has the advantage that it is not necessary to calculate the exact corner where is is, but it requires rotation independent centre definition
a different interpretation of the hotspot:
Observe, that a shape must always be in the first octant (x, y and z positive) regarding the coordinate system because of the way the coordinates are handled. This means that rotations can never be exact mathematical rotations around the source of the coordinate system there is always a translation added that brind the shape back into the first octant.
This added translation adds a major headache because it is dependent on the shape itself its size and shape. It is easy to calculate for cubes, but impossible to easily find out for the more obscure grids like the triangles.
To make the roation appear to be around the center the hotspot contains the added translation that shifted the whole shape into the first quadrant.
Now it becomes simple to rotate voxel spaces in space without knowing their exact position. All you do it rotate the position round the source