The solution is actually fairly transparent to users, since it is delivered in the installation directory:
See: CalculateVolume.py. In the CalculateVolume script, Esri provides the arcpy.SurfaceDifference_3d tool.
This tool uses the LAS point cloud and the user’s digitized shape to produce a DSM TIN surface and a DTM TIN surface. The DTM surface simply utilizes the points digitized by the user to create the base surface. The triangles of the DTM TIN stretch across the boundaries of the digitized polygon to create this base surface.
The DSM and DTM TINs are then input to the SurfaceDifference_3d tool. The resulting feature layer is dissolved into one measurement (a positive volume indicates the amount of volume).
Since the TINs are constructed with the user's digitized shape, it is very important that the user digitize as close to the base of the pile as possible. If one of the vertices is digitized up onto the side of the pile, then the resulting volume is lower than expected. This happens because if the Z-value for a badly digitized point is higher, and as a result, the underlying DTM does not reflect the ground. surface
It is not advisable to modify the script. If so, unexpected behavior may result, and Support Services is unable to troubleshoot the problem.