Cesium chloride has a cubic structure with eight $Cs$ atoms at each corner of each cube and one $Cl$ in the middle of the cube. It is stated in my solid state book that this crystal cannot be described by a BCC lattice due to the fact that the atoms are not equivalent whereas they should be by definition of a lattice.
But I have some trouble understanding this. I thought that a lattice is just a mathematical handiness of describing the real physical crystal. A lattice consists of points (whereas the atoms are not points) and is independent on the crystal. Why can’t we just put points on the atoms and describe the crystal completely this way? Why do the atoms need to be equivalent such that we can use a certain lattice ?