The difference is here anyway :
$ gmic sp lena r2dx 100 b 2 +elevation3d[0] 0.2 +elevation3d[0] -0.2 rm[0] c3d +3d. 120,0,0 +3d
On the left: a positive elevation.
On the right : a negative elevation.
Positive elevation means your 3d points are further (higher z-coordinates) when they have a high elevation.
Negative elevation means your 3d points are closer (lower z-coordinates) when they have a high elevation.