The center of the poly primitives is 0,0,0, but this will be reletive to the world co-ords with its origin on 0,0,0.

So if a cube 5,5,5 will have is local origin at 0,0,0 but in world it would be at 0,2.5,0. How to get MEL to pick this up would be getting the co-ords of the vertices. The cube would have 8 verts 4 top and 4 bottom each occupying its own space.

Enough theory... Have a look at this link:

https://www.robthebloke.org/mel/DATA_mesh.html