Because the ABC-structure admits of a c u b i c unit cell, it must belong to the Cubic Crystal System (also called Isometric System), and not, as was the case with the AB-structure, to the Hexagonal Crystal System. That's why we call the ABC-structure Cubic Close-packed. Of course we must imagine all the spheres in all the above Figures to be contiguous. We had built the ABC-structure by means of stacking layers of identical spheres, a stacking in a certain direction. But because the ABC-structure is cubic we can consider it at the same time as being stacked in the other three equivalent directions as well. These directions are parallel to the cube's body diagonals. So when you have built the structure, there is no way of telling from the finished structure that you stacked the layers perpendicular to a particular one of those diagonals, because the latter are wholly equivalent. You could have built the structure equally well by stacking the layers perpendicular to any of the four diagonals (these are the four axes of 3-fold symmetry).
Crystals of the metal Copper possess this Cubic Close-packed Structure (ABC-structure).
Now it is time to return to the NaCl crystal structure. This structure consists of two different kinds of chemical motifs, Sodium ions and Chlorine ions. These ions differ in size, or at least they behave as if they were of different size. So the structure as such cannot be an ABC close-packed structure. But if we look at the o r d e r i n g of either the Sodium ions or of the Chlorine ions, then we see that each of them is ordered according to a face-centered cubic lattice. So qua ordering the Sodium ions have arranged themselves according to the ABC-structure, and also the Chlorine ions are ordered according to the ABC-structure. Indeed the NaCl-structure can be seen as two interpenetrating face-centered cubic lattices. Those lattices are shifted with respect to one another by half a body diagonal of a face-centered cubic unit cell. To show this we repeat a Figure given earlier.