Section 17(4) in The Indian Postal Service (Group 'A') Rules, 1987
(4)If at the examination, sufficient number of candidates belonging to the scheduled castes or the scheduled Tribes is not available for filling all the vacancies reserved for them including those carried forward under sub-rule(3), the vacancies so remaining unfilled 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, and an equivalent number of vacancies shall be carried forward to the next examination and so on.
