sp3d2 results from the hybridization process. The hybridization process involves taking atomic orbitals and mixing these into hybrid orbitals. These have a different shape, energy and other characteristics than the component atomic orbitals.
The sp3d2 hybridization concept involves hybridizing three p, one s and two d-orbitals. This results in the formation of six different sp3d2 orbitals and these take on an octahedral arrangement. To classify something as octahedral, it must have six atoms, electron pairs or groups of atoms with a symmetrical arrangement around a single atom. This defines an octahedron's vertices. There are eight symmetrical faces on the molecule that 90 degrees between each of the atoms.