Are volumes in Houdini (both standard and VDB) vertex-centered or cell-centered?

When I programmatically create a .bgeo or .vdb volume and do not specify positioning, it automatically gets placed so that the corner vertex gets placed at (0,0,0). That seems to suggest that it is vertex-centered? Visually inspecting the voxels of a volume slice also makes it look that way. But, I can't find any documentation about this.

The samples of Houdini volumes are stored in the voxel centers. If you use volume slice, you will notice that it is half a voxel smaller than the original volume on each side as it is placing the generated points at the volume samples, not the corers of the voxels.