4 Answers
Let’s go into a deep dungeon of crystallography.
Easy answer: NaCl has two different ions (one at origin, the other at the center of unit cell), while the standard FCC is only one.
Complete answer: both NaCl and a standard FCC structure belong to the space group Fm-3m. And Fm-3m has two distinguished Wyckoff positions with a multiplicity 8. One at (0,0,0) and the other at (1/2,1/2,1/2) The difference between the two is that, in a standard FCC, only one of them are occupied, while in NaCl, both of them are occupied by crystallographically different ions.
Face-centered cubic (FCC, 23F) is one of 14 Bravais lattices. And among 230 space groups, 11 space groups (F23, Fm-3, Fd-3, F432, F4_132, F-43m, F-43c, Fm-3m, Fm-3c, Fd-3m, and Fd-3c) belong to the Bravais lattice of FCC.
It is hard to define the standard FCC structure, if you are imagining the crystal structure of copper metal (Cu) as that standard of FCC, here it is.
So let’s define this as the standard FCC structure. This structure belongs to the space group Fm-3m having the highest symmetry among the eleven FCC space groups.
So does NaCl. NaCl also belongs to the same space group Fm-3m. To see the difference, visit this page of the table of Wyckoff positions of the space group Fm-3m. You will see a gross list of coordinates. Ignore them. Just scroll down. You will see the simplest last two rows labelled with Wyckoff letters a and b. In NaCl, both of them are occupied by differentatoms. In Cu metal, only one of them are occupied.
We have so many natural substances that have two different ions/atoms occupying the two Wyckoff positions of multiplicity 8 of the space group Fm-3m. Since we have so many similar examples in nature, like NaCl, we decided to give a special name to such structures: Rock-salt structure. (most rigorous definition of the rock-salt structure)
Reference: https://www.quora.com/How-does-NaCl-structure-differ-from-a-standard-FCC-stucture
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