Section 17(3) in The Indian Postal Service (Group 'A') Rules, 1987
(3)If sufficient number of candidates who are members of the scheduled castes or the scheduled Tribes, are not available for filling all the vacancies so reserved, the vacancies not so filled shall be filled by appointing candidates not belonging to the scheduled castes or the scheduled Tribes, qualified by the standard prescribed by the commission for the service, an equivalent number of additional vacancies shall be reserved for candidates belonging to the scheduled castes or the Scheduled Tribes to be filled on the results of the next examination.
