[Avogadro-Discuss] What is occupancy means in CIF file

[Indra G. <ind...@ya...>]

Dear all,

In my input file to Avogadro, I have a CIF file that shown below:

...
...
loop_
_atom_site_label
_atom_site_type_symbol
_atom_site_fract_x
_atom_site_fract_y
_atom_site_fract_z
_atom_site_B_iso_or_equiv
_atom_site_occupancy
NA1  Na  0.0  0.5  0.0  2.50  1.0
NA2  Na  0.0  0.4335  0.75  4.95  1.0
SI1  Si  0.19897  0.42771  0.5418  1.37  1.0
SI2  Si  0.19656  0.19092  0.5454  1.49  1.0
SI3  Si  0.0871  0.3840  0.25  1.21  0.5
SI4  Si  0.0866  0.2280  0.25  1.32  0.5
AL3  Al  0.0871  0.3840  0.25  1.21  0.5
...
...

I am curious what is occupancy  means in CIF file. I confuse, why in the
same coordinate there are two or more atoms exist?
For example in above case, SI3 and AL3 are in the same place, but occ's
value are 0.5 for each.
It's different with SI1 that have occ's value is 1.

Could someone give me an explanation?

Warm regards,
Indra Gunawan