The Discrete ordinate (DO) radiation model is the most preferable and an attractive model as it allows the users to solve the problems ranging from surface-to-surface radiation to the participating radiation in combustion system over a wide range of optical thickness. Moreover, it provides a nice compromise between the accuracy and the computational cost. However, please check the Theory- & User-Guide of Fluent in order to get further details including the advantages and the limitations of the radiation models.