What is procedure allocation of cadre and posting of IAS officers?

I would like to know how cadre allotment is done to candidates selected in IAS and what is the procedure adopted for their initial posting i.e., how is it decided as to who would be posted where? Whether only merit during the training period is considered or is there also some other criteria? Is there any written policy for this. If yes, where can I get it from?
An early response will be highly appreciated.

Vijaya Lakshmi