Exactly as described in the manual (p. 102). 'Occupation number i.e. chemical occupancy × site multiplicity (can be normalised to the multiplicity of the general position of the group).' You can get the occupancies from whatever source you use for the initial model (journal article, ICSD,...) and multiplicities from the International tables for crystallography.

Dear Alexander thanks for your kind reply. When I load the cif file from ICSD it is giving 0.5, 0.25, 0.25, 1.0. but the .dat file of it is showing all values are 1,1,1,1.