You always start out with local coordinates, e.g. (-1., 1., 1.) for the corner of a box. And to get to the world-space coordinates you transform these coordinates with the net transform of the box:
local_point = Point3(-1., 1., 1.)
mat = model.get_net_transform().get_mat()
world_point = mat.xform_point(local_point)
Do you still remember this, where we talked about global coordinates, net transforms and all that 
?
The code I added to that post should still be relevant, I think.