In the present study an attempt to clarify discrepancy between experimental and theoretical results for preference substitution of d or b indium cation sites by tin is performed. For this purpose, the density functional calculations for a number of oxidized and reduced states of ITO with different configurations for tin substituted cation sites are carried out. For reduced states the probability for tin substituents to occupy the b positions is significantly larger than the same value for the d sites. These results indicate that the more favorable occupancy of either b or d cation positions by tin substituent depends on the way of the defect formation.